Abstract: The continuous cooling transformation curves of under-cooled austenite in Nb-V microalloyed spring steel 60Si2CrVAT were measured by using a DIL805A quenching deformation dilatometer.The microstructure changes of under-cooled austenite in the tested steels at different cooling rates were studied by the metallographic method coupling with the microhardness method.The effect of Nb content on continuous cooling transformation of the tested steels was analyzed.The experimental results show that with the increase in Nb content,the high-temperature and medium-temperature phase transitions were promoted,the range of pearlitic transformation was widened,and the temperature of Ms point decreased from 284.2℃ to 258.3℃.The cooling rate for ferrite disappearance increased from 0.3℃·s^-1 to 0.5℃·s^-1,for pearlite disappearance from 2℃·s^-1 to 3℃·s^-1,and for bainite disappearance from 3℃·s^-1 to 5℃·s^-1.The cooling rate for bainite transformation occurrence increased from 0.4℃·s^-1 to 0.6℃·s^-1,while for martensite occurrence kept the same cooling rate of 1℃·s^-1.