Abstract: Objective To explore the effect of olanzapine on weight in schizophrenic patients. Methods Totally 27 patients with genotype TT and TC of 5-hydroxytryptamine type 2C receptor(5-HTR2C)(-759 C>T)were treated by olanzapine. The changes of body mass, Qvetelet body mass index, blood glucose and blood lipid were compared before treatment and 12 weeks after treatment as well as scores of Positive and Negative Symptom Scale(PANSS)before treatment and 2, 4, 8, 12 weeks after treatment were compared. Results After 12 weeks of treatment, the body weight of patients increased significantly when compared to that before treatment(P<0.01), while Qvetelet body mass index increased by(7.00±2.66)%, but there was no significant difference before and after treatment(P>0.05). There were no significant differences in blood glucose, cholesterol, triglyceride, high density lipoprotein and low density lipoprotein before and after treatment(P>0.05). After 2 weeks of olanzapine treatment, the scores of positive symptoms, negative symptoms and general symptom of PANSS as well as total score were significant different from those before treatment(P<0.01). Conclusion Olanzapine can effectively improve the mental symptoms of patients with schizophrenic. Medical staffs should pay attention to the adverse reactions caused by olanzapine such as weight gain, and actively take early preventive measures to improve the therapeutic compliance and quality of life of patients.