The study aims to compare the effectiveness of high-calorie, high-fat diets with different glycemic index, on some indicators related to diabetes and oxidative stress in children and adolescents with cystic fibrosis..In this study, which is a double blind randomized controlled trial, study population are children and adolescents with cystic fibrosis who will come to cystic fibrosis clinic in Children Medical Center. After Cystic Fibrosis in patients was approved by pulmonogists, patients will be referred to nutrition team in order to get proper diet.After assessment for inclusion criteria, including the age range between 2-18 , children and adolescents with cystic fibrosis whose disease is approved by sweat test or genetic test by the clinician, willingness to cooperate and participate in the study and not having no entry criteria like:developing other chronic diseases, severe liver disease (cirrhosis), hospitalization at baseline or one month earlier in the hospital, undergoing surgery in the next three months or a month before the start of the study (with the exception of dental surgery).Benefits and risks will be explained to parents and children explanation will be given to parents and children if they were willing to cooperate the consent form will be signed by parent or children.In second visit 10 cc of blood for biochemical tests will be taken from patients, general information and individual characteristics forms as well as dietary recall questionnaire will be filled. Indicators related to anthropometric characteristics before the intervention, including height (height gauge with accuracy of 0.5 cm), weight (by scales Seca carefully 100g) will be measured and BMI (Body Mass Index)will be calculated by formula and will be recorded in general informatin form.Selection of one of the two regimes was through randomization method by Stratified Randomized Block Design in terms of sex and age and patients will be entered into one the two groups high calorie, high fat or high calorie, high fat diet with low glycemic index( sample size of each group=21). The number of food units per serving which is written in paper, and Exchange List tab and nutritional advice and allowed foods(for Group II) will be given to each of the patients in both group. Patients will be assessed via telephone whether they adhere to their diets every two weeks within three months. The researcher will be available to answer any questions and fix problems with the regime through this time.In last visit after three months from the begining of the intervention 10 cc of blood for biochemical assessment will be taken and height and weight and BMI of the patients after three months of intervention will be recorded in general information form.