Background: Gastric size determines the amount of dietary intake which in turn determines energy intake. Body weight loss depends on negative energy balance. Objectives:The primary purpose of the present study was to assess body weight loss for 36 months in patients who underwent gastric sleeve surgery. A secondary purpose was to examine the impact of gender and age on weight changes before and after surgery. Methods: A total of 309 patients were enrolled sequentially. Age, body weight, gender, BMI, and comorbidity were assessed before and after gastric sleeve surgery for 36 months. Results: Results showed that body weight before and after surgery was influenced by both age category and gender. It was also noted that the rate of body weight reduction was faster during the 1 st 12 months compared to the 2 nd 12 months. Conclusions:Based on the results of the present study, it can be concluded that prior and post gastric sleeve surgery body weight is influenced by age and gender. The rate of weight reduction after gastric sleeve surgery is faster during the first 12 months, compared to the second 12 months. Gastric sleeve affected body loss and that can be considered as effective treatment for obesity and obesity related diseases.