Objective To determine the association between weight gain induced by atypical antipsychotic and the polymorphisms of MC4R gene rs12970134.Methods 62 patients who had weight gain more than 7% of their pre -drug body weight were selected as study group,and 62 patients who had weight gain less than 7% of their pre -drug body weight were selected as control group.The polymorphism of MC4R gene rs12970134 was analyzed by using polymerase chain reaction and directly sequencing technology.Results There were no significant differences in the frequency of MC4R gene rs12970134 genotypes and alleles between the two groups(χ2 =0.648,P =0.723;χ2 =0.679,P =0.410).While after the treatment with atypical antipsychotic,the weight gain degree in patients with GG genotypes was less than patients with GA /AA genotypes[(22.18 ±0.33)kg/m2 vs.(23.53 ±0.58)kg/m2 ](t =-2.167,P =0.032).Conclusion The polymorphisms of MC4R gene rs12970134 maybe affect the weight gain degree in patients after treatment with antipsychotic.