Introduction: obesity is currently one of the main clinical complains. Metabolic syndrome is a complication of obesity and is a combination of central obesity, dyslipidemia, glucose metabolism alteration, hypertension, and insulin resistance, that may begin in childhood and progress through adulthood and increases the risk of diabetes and cardiovascular diseases. Risk of obesity and its complications is higher in obese children than normal weight children. Preventing obesity and metabolic syndrome in children prevents early occurrence of diabetes and cardiovascular disease. Sirtuins are deacetylase enzymes which act using NAD and effect gene expression by deacetylation of histones. SIRT1 is one of the sirtuins that has been shown to be associated with metabolic syndrome, insulin resistance and diabetes. This enzyme has been shown to be reduced in obesity and insulin resistance and is suggested as a marker and target for metabolic disorders. Because of the protective effect of SIRT1, its reduction in obesity may be responsible for the complications of diabetes and therefore induction of its expression may be effective in prevention of obesity complications. Metformin has been shown to be able to induce SIRT1 activity but this effect has not been previously studied in children and adolescents. The aim of this study is to assess the effectiveness of metformin, which is an approved and widely used medication, on SIRT1 expression. 1) objectives: Main objective: determination of the effect of metformin on sirtuin gene expression in peripheral blood mononuclear cells in 8-18 year old children and adolescents who are referred to H. Aliasghar hospital. Minor objectives: a- determination of sirtuin gene expression in obese children before administration of metformin; b- determination of sirtuin gene expression in obese children after administration of metformin; c- comparison of sirtuin gene expression in obese children before and after administration of metformin; d- determination AMPK levels in obese children before administration of metformin; e- determination AMPK levels in obese children after administration of metformin; f- comparison of sirtuin gene expression in obese children before and after administration of metformin; g- comparison of BMI and waist circumference in obese children before and after administration of metformin; h- comparison of metabolic parameters in obese children before and after administration of metformin; i- comparison of HOMA-IR in obese children before and after administration of metformin 2) Design: This study is designed as a double blind placebo-controlled randomly-assigned clinical trial 3) setting and conduct: all the participants undergo clinical evaluation and height, weight, waist circumference, hip circumference and blood pressure is recorded for them. Blood sample is withdrawn from participants before the beginning of intervention and after finishing it, and routine biochemical tests including blood glucose, serum insulin, and lipid profile including total triglyceride, total cholesterol, LDL and HDL are performed. These measurements are performed by calorimetric methods using conventional commercial kits. HOMA-IR is calculated based on data from fasting blood glucose and insulin levels. 5 ml of each blood sample is processed, plasma and PBMC are separated and kept frozen after separation of plasma and PBMC, until later analysis. SIRT1 gene expression is analyzed by real-time PCR with SYBR-green using gene specific primers after extraction of RNA and synthesis of cDNA. Plasma insulin and AMPK levels are measured by ELISA method. 4) Participants and criteria: a total of 60 participants from obese children and adolescents who are referred to outpatient clinic of Hazrat Aliasghar hospital are included and randomly assigned to two 30-cases groups. a. inclusion criteria: children who have BMI more than 95th percentile (regarding their age and gender), absence of any illnesses or history of it, not consuming medications including antihypertensive drugs or any other medication known to affect metabolism of lipid or carbohydrate b. exclusion criteria: major illnesses including endocrine, cardiovascular, renal, etc. or having a recent history of these diseases, BMI lower than 5th percentile or between 85th and 95th percentile. Intervention: 30 participants receive metformin with the dose of 1000 mg/m2 body surface. The other group receives placebo. The total intervention duration is 3 months. Main outcome measures: weight, BMI, waist circumference, hip circumference, waist to hip circumference ratio, fasting blood glucose, total triglyceride, LDL, HDL, insulin, sirt1 gene expression, AMPK