Abstract: The CCT curve of 3CrMnMoVS steel has been mersured, and it is used to analyse the optical and electron-microstructure of the steel, which was continuouslly cooled at a moderate cooling rate of 200-1000℃/h. The marphology observation and electron diffraction analysis show that under the cooling rate greater than 0.72℃/s the microstructure mainly consists of lath matensite, in contrast after air cooling the morphology indicates the product of intermediate temperature, e.g. the fine mixed structure of B_g+B_u+B₁ with a few dispersive VC in it.