Objective · To investigate the effect of modified electroconvulsive therapy (MECT) on oxidative stress parameters in patients with bipolar disorder.  Methods · Forty-three patients with bipolar disorder (case group) were enrolled that received MECT intervention for 6 weeks, and 49 healthy volunteers (control group) were recruited. Chinese versions of the 17 items Hamilton Depression Rating Scale (HAMD-17), Young Mania Rating Scale (YMRS) and Clinical Global Impression-Severity Scale (CGI-S) were used to assess the efficacy and side effects at baseline and after 6 weeks of treatment. The plasma levels of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px) and malondialdehyde (MDA) were measured at baseline and after 6 weeks of treatment to assess the level of oxidative stress.  Results · The serum MDA and GSH-Px levels of the case group were higher while the serum SOD levels of the case group was lower than that of the control group, and there was no significant difference in the serum CAT levels between two groups at baseline. MDA levels were higher in manic states than in depressed states, and they were positively correlated with the CGI-S scores. After MECT treatment, the CGI-S scores of patients decreased significantly, and the plasma MDA levels decreased significantly in manic and depressive states, but there was no change in other oxidative stress parameters.  Conclusion · There was oxidative stress damage in patients with bipolar disorder, and the severity of the disease varied with the degree of damage. MECT improved the symptoms of the disease and decreased the level of plasma MDA, while there was no effect on the anti-oxidation index.