Alzheimer's disease is the most common form of age related cognitive impairment. Aim of the present study was to see the effect of vitamin D on cognitive function in elderly. The study was conducted in Department of Physiology, King George's Medical University (KGMU). A total of 80 subjects were enrolled based on Mini Mental State Examination (MMSE) score < 24 and vitamin D deficiency. They were divided into two groups as Group A (case) and Group B (control), each group having 40 subjects. Intervention (Vitamin D supplementation) was given in Group A. The assessment of dementia was done by Mini Mental State Examination (MMSE) score. Every subject was clinically evaluated and estimation of vitamin D was done by direct ELISA kit. Gender, weight, height, BMI, residence and education were also similar between two groups. A significant (p=0.0001) change in MMSE score was observed in both Group A and Group B from baseline to 3 & 6 months and from 3 to 6 months, however, mean change was higher in Group A than Group B. In conclusion, vitamin D supplementation caused significant improvement in the cognitive performance in subjects with senile dementia.