Metformin is one of the most widely prescribed medications for the treatment of type 2 diabetes (T2DM) but metformin response varies greatly between individuals,polymorphisms in genes that regulate metformin’s pharmacokinetics and pharmacodynamics may be to blame for these phenotypic differences in drug disposition and action. Inthis study, we investigated the association between 420del (rs72552763) organic cation transporter 1 (OCT1) polymorphism and therapeutic response to metformin in type 2 diabetic patients, A total of 200 diabetic patients enrolled in this cross-sectional study, patients were chosen when they visit outpatient clinic for medical care. Fasting serum glucose, fasting serum insulin and HbA1c, which indicate metformin response were measured. The result of the study indicate there was a significant association between the presence of 420del (rs72552763) in diabetic patients and the variability response to metformin, the multinomial logistic regress analysis indicates that most patients with the reference allele (wild type) and heterozygous alleles of OCT1 (420del) had statistically significant metabolic responses to metformin, whereas patients with mutant alleles had less or no response. In conclusion, Genetic polymorphism of OCT1transporter 420del (rs72552763) contributed to the variability of therapeutic response to metformin treatment.