To learn capability of grass carp(Ctenopharyngodon idellus)to cross velocity barrier,laboratory experiments were conducted to test burst-coast swimming performance of grass carp through startle response.In this study,the tested fish were divided into three groups according to their body length:(8.47±0.73)cm(small-size),(17.93±1.27)cm(mid-size) and (51.24±3.24)cm(large-size).The experiments were carried out in an environment with natural light and water temperature of(20.00±1.50)℃.The results showed that the highest absolute sprinting speeds of fish were(1.449±0.424)m/s(achieved in 0.294 s),(2.359±0.434)m/s(0.294 s) and (2.899±0.457)m/s(0.378 s)for small-size,mid-size fish and large-size fish respectively; the relative sprinting speeds were(17.099±5.009)BL/s,(13.156±2.418)BL/s,and(5.659±0.891)BL/s,respectively.Fish were observed to slow down to coast by keeping their body straight(no tail-beat)after fish reached their highest sprinting speeds.Absolute sprinting speed of grass carp was found to increase with body length(P>0.05),while relative sprinting speed decreased as body length increased.Relative sprinting speed of small-size fish was significantly higher than that of large-size fish(P<0.05).Absolute acceleration of fish was not affected by size(P>0.05); while the absolute coast deceleration of large-size fish was significantly higher than those of smaller size fish(P<0.05); and the relative burst acceleration and relative coast deceleration of large-size fish were both significantly lower than that of small-size fish(P<0.05).