Abstract: To investigate the effect of dissolved oxygen (DO) level on swimming performance of juvenile Chinese bream (Parabramis pekinensis) n=32, body mass: 4.700.11 g and Qingbo (Spinibarbus sinensis) n=32, body mass: 3.260.06 g, we measured the critical swimming speed (Ucrit) and swimming metabolic rate (MO2) and calculated the resting metabolic rate (MO2rest), maximum metabolic rate (MO2max), metabolic scope (MS), and cost of transport (COT) during swimming at 25℃ and DO 8 (100% saturation), 4, 2, and 1 mgO2/L. Ucrit values of Chinese bream and Qingbo were 8.47 0.19 vs 9.49 0.14, 7.40 0.17 vs 9.40 0.28, 5.64 0.16 vs 7.51 0.21 and 4.02 0.23 vs 4.75 0.13 BL/s at above different DO levels, respectively. Ucrit decreased significantly with DO in both fish species (P0.05) except that of Qingbo between 4 and 8 mgO2/L. Ucrit of Qingbo was significantly higher than those of Chinese bream under all DO levels (P0.05). Both MO2max and MS decreased significantly with DO from 8 to 1 (P0.05) while MO2rest showed a significantly decrease till DO was below 1 mgO2/L. In comparison, MO2 increased significantly with Ucrit (P0.05). COT of both fish species decreased with Ucrit within lower swimming speed range but remained unchanged at high Ucrit range. COT of Qingbo was lower than that of Chinese bream (P0.05). In addition, COT decreased with DO, suggesting that down-regulation of the physiological processes under hypoxic conditions. When DO decreased from 100% to 50%, Chinese bream showed a 13% decrease in Ucrit due to the decreased respiratory capacities (as suggested by a 32% decrease in MO2max) while Qingbo showed no significant decrease in Ucrit despite a 20% decrease in MO2max due to the decrease in maintaining metabolism and COT. However, when DO decreased to 12.5%, both Qingbo and Chinese bream showed similar decreases in Ucrit compared to those under 100% DO (53% vs 50%). This observation suggested that the swimming performance varied differently with the DO decrease between Chinese bream and Qingbo and was closely related to the metabolic strategy of different fish species.