Introduction: Psychiatric disorders such as schizophrenia require appropriate treatment. Second-generation antipsychotics, including olanzapine, are administered as an appropriated treatment for these disorders. However, this drug has several complications, such as obesity and increased waist circumference which reduces the acceptance of the treatment. Objectives: The aim of this study was to investigate the effects of vitamin D3 therapy on waist circumference caused by olanzapine. Patients and Methods: In this randomized clinical trial, 50 hospitalized women with psychiatric disorders receiving olanzapine were divided into two groups of intervention and control groups. Before the intervention, the waist circumference, hip circumference, and waist-hip ratio were measured. Then, for 12 weeks, the intervention group received vitamin D3 and the control group received paraffin as the placebo, one pearl every 2 weeks. Waist circumference, hip circumference, and waist-hip ratio were re-measured and the collected data were analyzed by pair t test. Results: Our findings showed that there is a significant decrease in waist circumference and waist-hip ratio in intervention group (P < 0.001). However, these changes were not significant in the placebo group. Hip circumference was not changed significantly in both groups. Conclusion: Vitamin D can be administered concomitant with 0lanzapine to improve side effects of olanzapine such as increasing waist circumference.