Background: Vitamin D can be effective in treating some symptoms of schizophrenia, and with better control of vitamin D levels and supplementation, there is a potential for reducing this disease. But due to small number of studies, more studies are needed in this area. The aim of this study was to determine the efficacy of vitamin D as an adjunct to the positive, negative, and cognitive symptoms of people with chronic schizophrenia. Methods: The present study is a double-blind clinical trial. A total of 30 patients who were eligible for inclusion criteria were selected randomly. The Positive and Negative Syndrome Scale (PANSS) scale was used to determine the status of positive symptoms, negative symptoms and general psychopathology, and The Mini-Mental State Examination (MMSE) to determine cognitive status when entering the study and at the end of the eighth week. Results: There was no significant difference between the two groups in terms of mean negative symptoms before and after the study. The overall symptoms score based on PANSS did not show a significant difference before and after treatment with vitamin D (P=0. 8). There was no significant difference between the two intervention and control groups regarding the mean of general symptoms before and after the study. Also, there was no significant difference between the two intervention and control groups before and after the study in terms of mean total score based on PANSS. Conclusion: Adding vitamin D to the standard treatment of schizophrenia did not have a significant effect on the improvement of positive, negative and invasive symptoms and general symptoms.