Background. TO evaluate the effects of CoQ10 on myocardial protection in patients undergoing cardiac valve replacement and direct scavenging free radicals activity in vitro. Methods. Twenty-four patients were randomly divided into two groups. Twelve patients in the CoQ10 group received intravenous and intracoronary CoQ10-treated "round the operative period". Twelve patients in the control group received no CoQ10-treatment. Measures. Plasma malondialdehyde (MDA) concentration, erythrocyte superoxide dismutase (SOD) activity, and serum cardiac isoenzyme of creatine kinase (CK-MB) were measured in the perioperative and postoperative period. The effects of CoQ10 direct scavenging free radicals were determined with electron spin resonance (ESR) and spin-trapping techniques by an in vitro study. Results. Plasma MDA concentration and serum CK-MB levels in the CoQ10 group were significantly lower than those in the control group. Erythrocyte SOD activity in the CoQ10 group was significantly higher than that in the control group. CoQ10 showed an obvious hydroxyl radical scavenging activity, but it could not scavenge superoxide anion radicals. Conclusions. These findings demonstrated that the use of intravenous and intracoronary CoQ10-treatment may play a more beneficial protective role during cardiac valve replacement through its antioxidant properties and membrane stabilization, as well as through its ability to scavenge hydroxyl radicals directly.