BACKGROUND Non-alcoholic steatohepatitis (NASH) is a serious comorbidity in patients with obesity and because of the high risk of cirrhosis and the extreme mortality rate of NASH, approaching effective treatment methods, and improvements are crucial. Following few studies comparing the impact of laparoscopic gastric bypass (LGB) and laparoscopic sleeve gastrectomy (LSG) surgery on NASH, our study was conducted to assess the effect of these two surgical methods separately in patients with NASH through ultrasonography, and concurrently, on other factors such as lipid profiles and blood pressure to reduce the complications of complex obesity surgeries on patients and also provide a solution to prevent NASH-related cirrhosis. METHODS This study was performed at Imam Khomeini Hospital Complex; Vali-e-Asr Hospital affiliated to Tehran University of Medical Sciences.All patients with obesity who had undergone bariatric surgery at Vali-e-Asr Hospital during 2017 and 2018 were included in this prospective cohort study. Weight, body mass index (BMI), blood pressure, Alanine transaminase(ALT), Aspartate transaminase(AST),lipid profile and Fasting blood sugar(FBS)were evaluated before and one year after surgery. The patients underwent an ultrasound examination before surgery to determine the fatty liver grade. The type of operation (sleeve or bypass) was governed by the patient him/herself after receiving thorough clarifications about the available methods, their complications, and expected outcomes. In addition, by the separation of fatty liver grading with ultrasound modality, the correlation between ultrasound grading, laboratory results, and the rate of weight loss in patients who undergone both sleeve and bypass surgeries were evaluated and compared during a one-year follow-up. RESULTS In this study, 44 patients were included. 22 patients underwent laparoscopic sleeve gastrectomy (LSG), and 22 patients underwent laparoscopic gastric bypass (LGB) surgery. The mean age of the patients was 40.45 ± 12.01 years. 35 patients (79.5%) were women, and 9 patients (20.5%) were men. Most patients (81.8%) had grade I and II in terms of preoperative liver ultrasonography results. Bariatric surgery (LSG and LGB) greatly enhanced NASH's hepatic status in liver ultrasonography. Since the liver status of patients with preoperative ultrasonography was not significantly different between the two groups, there was no substantial distinction among the two groups in this regard, postoperatively. Weight and BMI, lipid profile, liver enzymes, FBS, and mean arterial blood pressure (MAP) were significantly reduced individually in both groups and all patients one year after surgery. None of the postoperative variables and their modifications had a prominent difference between the two groups except for High-density lipoprotein(HDL)level after surgery. The postoperative HDL was considerably higher in the LGB group (p = 0.014). However, the changes in HDL were not statistically different between both groups. The levels of AST, ALT, total cholesterol, LDL, and FBS were associated with the NASH grade. CONCLUSION Both types of LSG and LGB bariatric surgeries have been shown to significantly reduce BMI and improve lipid profiles, liver enzymes, and blood glucose levels in patients one year after surgery. The NASH status was also ameliorated considerably. The two types of surgery were not remarkably different in these modifications. Besides, there was a significant correlation between AST, ALT, total cholesterol, LDL, and FBS levels with the NASH grade.