The aim of this study was to investigate whether coenzyme Q10 (CoQ10) has an antifatigue effect in mice. ICR male mice were orally given CoQ10 in the form of Bio-Quinone (Pharma Nord, Vejle, Denmark) at doses of 0, 1.5, 15, or 45 mg/kg/day for 4 weeks. Mice were made to perform swimming exercise with loads attached to their tails, corresponding to approximately 5% of their body weights, and the total swimming time until exhaustion was measured. Furthermore, the post-exercise concentration of serum urea nitrogen (SUN), pre-/post-exercise and post-rest concentration of lactic acid (LA), and pre-exercise hepatic glycogen were determined. Mice treated with CoQ10 showed a significantly prolonged exhaustive swim time (15 mg/kg/day; P < .05), increased liver glycogen contents (15 and 45 mg/kg/day; P < .01 and P < .05, respectively), and decreased SUN levels (1.5, 15, and 45 mg/kg/day; P < .01) compared to control animals. The LA level was not significantly changed. These results suggest that CoQ10 improves swimming endurance and has an antifatigue effect.