Abstract: Temperature fluctuations exist on the slab surface due to unstable heat flux during repeated contact and departure between the hot slab surface and the relatively cold supporting rollers in the secondary cooling zone. Considering thermal oscillations,hot ductility test under different thermal conditions was carried out on J55 microalloyed steel. The results show that at a temperature higher than 850℃,the hot ductility under the oscillating condition is lower than that under the isothermal condition,but it is on the contrary when the temperature equals or falls below 850℃. By analyzing results from experiment and from literature comprehensively,for the microalloyed steel,it is inferred that there exists a critical temperature a bit higher than the Ae₃ temperature. Above this critical temperature the hot ductility under the isothermal condition is higher than that measured by the more accurate oscillating way,while below it the result is opposite. This conclusion is meaningful to determine the straightening temperature for microalloyed steel production during continuous casting.