Background: The purpose of this study was to evaluate the effects of short term (12 day) Coenzyme Q10 (CoQ10) supplementation on blood oxidative stress biomarkers and physical performance in young swimmers. Methods: Sixteen young swimmers (7 males and 9 females, aged 15.13±0.96 years) who were involved in a sport hero project at Chiang Mai province participated as subjects during this 22 day study. Regular training for swimmers was carried out for the first 9 days. Supplementation with 300mg CoQ10 (Soft gel; Swanson Ultra, USA) was then administered daily for 12 days. Blood samples were collected in EDTA-anticoagulant tubes before (days 1 and 9 of the control period) and after the 12 day supplementation period (day 22). Plasma was separated and used for the determination of malondialdehyde (MDA), nitric oxide (NOx), protein hydroperoxide (PrOOH), total antioxidant capacity (TAC), and CoQ10; whereas reduced glutathione (GSH) was measured in erythrocytes. Exhaustive exercise time was evaluated before (days 1 and 9) and after-supplementation (day 22) on a mechanical treadmill using a modified Bruce protocol. Swimming speeds for both 100 and 800 meters were also recorded. Repeated measurement and Bonferroni correction were used for statistical analysis (p = 0.05). Results: Over the course of the 9 day control period before supplementation, all parameters (MDA, NOx, PrOOH, CoQ10, GSH, TAC, treadmill exhaustion time, and swimming time for either 100 or 800 meters) did not differ (p > 0.05). After CoQ10 supplementation, the levels of plasma MDA, NOx, and PrOOH were significantly decreased when compared to the pre-supplement period (p < 0.05). Moreover, plasma CoQ10 (2.34±0.78 μg/ml vs 1.10±0.38 μg/ml) and erythrocyte GSH (14.51±1.99 vs 8.90±1.84 μmol/g Hb) significantly increased (p < 0.05). Maximal treadmill time was increased significantly (22.92±4.89 mins) when compared to pre-CoQ10 supplementation (p < 0.05), whereas, the swimming time improved significantly for 100 meters when compared to pre-supplementation (p < 0.05), with no difference in swim time recorded for 800 meters (p > 0.05). Conclusion: Twelve days of CoQ10 supplementation reduces oxidative stress, improves running time until exhaustion, as well as short distance swim sprint time, within a sample of young swimmers.