To determine the effect of vitamin D supplementation on post-operative pain and analgesic requirement in brain tumor surgery.A total of 60 patients with vitamin D serum level of ≤ 20 ng/dL were randomly assigned to two groups equally. The study group (n=30) received intramuscular injection of 300,000 IU vitamin D before surgery.Preoperative serum level of vitamin D was 15.9±3.8 ng/dL and 14.5±3.6 ng/dL in the study and control groups, respectively (P= 0.13). Serum level of vitamin D on day 5 of surgery was 22.5±4.3 and 13.7±3.8 in the study and control groups, respectively (P<0.001). A percentage of 50% had pain scores above 4 on the first postoperative day, which decreased with time. The median (IQR) of the visual analogue scale (VAS) score during the three post-operative days was 3(5), 3(5), 1(3) and 5(7), 2(5), 1(3) in the study and control groups, respectively, with no significant difference. There was no difference in analgesic consumption between the two groups. Analysis through the Generalized Estimating Equation model indicated that patients who had received vitamin D for a longer time before the operative time had an insignificantly less pain score.Based on the study results, half of our patients reported moderate to severe pain scores on the first day after surgery. The pain in the study group was insignificantly less than that in the control group, but it seems that chronic high level of vitamin D may lead to promising results.