Abstract: Objective: To determine the effect of cardiomyopeptidin on transient outward potassium current (Ito) of rat ventricular myocytes and its action mechanism on the ion channels of myocardium. Methods: Single ventricular myocytes of rats were obtained by enzymatic dissociation method. The whole-cell patch-clamp recording technique was used to record the change of transient outward potassium current (Ito) by different dosage of cardiomyopeptidin. Results: Cardiomyopeptidin decreased Ito in a dose-dependent manner. Cardiomyopeptidin in dose of 10, 50, 100, 250 and 500 mg/L decreased Ito (%) by 4, 13, 22, 32 and 38 respectively. Cardiomyopeptidin 50 mg/L moved down the current density-voltage curve of Ito, but the shape of the curve had no changes. Cardiomyopeptidin 50 mg/L did not change the steady state activation curve of Ito. Conclusions: Cardiomyopeptidin decreases the Ito of rat ventricular myocytes, which might be one of the mechanisms of its antiarrhythmic effect.