Abstract: The aim of this study was to evaluate the body mass index dependent metabolic syndrome in severe mental illness patients in Gorgan. A total of 267 severe mental illness patients took part in this study. The prevalence of metabolic syndrome in severe mental illness patients in different body mass index were 6.67, 24.09 and 53.06%. There were significant differences in the mean of waist circumference, HDL-cholesterol, triglyceride and fasting blood glucose in subjects with metabolic syndrome in different body mass index when compared with subjects without metabolic syndrome (p<0.05). The prevalence of high fasting glucose, low high density lipoprotein-cholesterol, high triglyceride levels, high waist circumference and high blood pressure were 14.23, 38.57, 41.57, 32.96 and 5.24%, respectively. It shows that high triglyceride levels (41.57%) and Low HDL-cholesterol levels (38.57%) were the most frequent characteristics in comparison to other metabolic components. Our results show that, 26.96, 31.08, 21.35, 15.35 and 5.25% of subjects had zero, one, two, three and four criteria for metabolic syndrome, respectively. These results show that the prevalence of metabolic syndrome in severe mental illness patients in Gorgan is increased with elevated body mass index. The results of this study suggest that mental illness patients are at risk of metabolic syndrome, when the rate of body mass index increases. Risk factors such as high triglyceride level and low HDL-cholesterol may play an important role in the occurrence of metabolic syndrome in severe mental illness patients.