In this study, the effects of vitamin D and placebo on ultrasonographic and laboratory criteria for fatty liver and some biochemical parameters of blood are compared in overweight and obese children aged 12 to 18 years. The population involves schoolchildren in Birjand among whom a total of 120 will be selected by multi-stage sampling method and will be assigned into control and intervention groups via simple random method (60 in each group). Major inclusion criteria include overweight and obese children based on Centers for Disease Control and Prevention of the United States; children with fatty liver based on ultrasound criteria; and age between 12 and 18 years. Major exclusion criteria include children with underlying conditions including thyroid dysfunction, diabetes and Cushing's syndrome, as well as the chronic consumption of any drug. The study is multi-centered and in the second phase of clinical trials. The intervention group receives 50000 international unit vitamin D as pearl on a weekly basis for 12 weeks; the control group will receive placebo in the same color and dose for the period of time. Before the intervention and at the end of the 12-week intervention, a set of tests were performed for liver enzymes functions (aspartate aminotransferase, alanine aminotransferase and alkaline phosphatase), some biochemical parameters of blood (blood lipids, uric acid, albumin, insulin and fasting blood glucose ), as well as complete blood count with differential, C-reactive protein, vitamin D, and liver ultrasonography.