BACKGROUND Vitamin D supplementation has been shown to decrease insulin resistance through which it might cause fatty liver. Fatty liver increasingly results in type 2 diabetes mellitus (T2DM). Insulin resistance and fatty liver are particularly closely related. The aim of present study is to examine the effect of vitamin D supplementation on blood sugar and different indices of insulin resistance in patients with non-alcoholic fatty liver disease (NAFLD). MATERIALS AND METHODS This randomized placebo-controlled clinical trial was conducted on 60 patients with NAFLD, who were divided equally into intervention and control groups. Patients in the intervention group received vitamin D3 (50,000 IU) and patients in the control group received placebo capsules every week for 10 weeks. Blood sugar, homeostatic model assessment-insulin resistance (HOMA-IR), and homeostatic model assessment-beta cell (HOMA-B) were checked at baseline and after 10 weeks of the intervention. Adjustment for variables was performed by analysis of covariance (ANCOVA). RESULTS Vitamin D supplementation resulted in increased serum 25-hydroxy vitamin D [25(OH) D] concentration in the intervention group compared to the control group [+68 (12) vs. -1.9 (2.44); P = 0.001]. Intake of vitamin D supplements led to a marginally significant decrease in fasting blood glucose [FBS: -12 (4) in the intervention group compared to - 3 (2) in the control group; P = 0.055]. Also, HOMA-IR decreased in the intervention group compared to the control group [-1.75 (0.23) vs. 0.12 (0.41); P = 0.066]. CONCLUSIONS Vitamin D supplementation resulted in decreased HOMA-IR and FBS concentration in patients with NAFLD; however, it did not affect the insulin level and HOMA-B significantly.