The 12Ni3CrMoVA steel was selected to investigate the effect of tem-perature, strain rate and microstructure on yield strength of bcc metals. The therma-lly activated parameters, such as activation energy and volume, were measured andcalculated. According to the P-N mechanism, the plastic deformation of bcc metalswas analysed by the core structure of screw dislocation, the theoretical predictionwas in good agreement with experimental results. It is explained that the "turningpoint" in curve of effective stress against temperature was caused by a double cam-el-hnmp--shape of Peierls potential energy, which resulted in a new pair of doublekinks forming in the front of original one. The microstructure can only affect on athermal stress, but not affect on thermally activated movement of dislocation.