In order to perform numerical simulation of forging and establish the hot formation processing parameters for 42CrMo steel, the compressive deformation behaviors of 42CrMo steel were investigated at the temperatures from 850 β’ C to 1150 β’ C and strain rates from 0.01 s -1 to 50 s -1 on Gleeble-1500 thermo-simulation machine. It was found that the flow stress of 42CrMo steel is evidently affected by both deformation temperature and strain rate, i.e., the flow stress decreases with the increase of deformation temperature and the decrease of strain rate, which can be represented by a Zener-Hollomon parameter in an exponenttype equation. For the relatively high temperature and low-strain rate, a typical flow stress curve is composed of four stages: stage I (work hardening stage), stage II (transition stage), stage III (softening stage) and stage IV (steady stage). While for the relatively low temperature and high-strain rate, stage III (softening stage) and stage IV (steady stage) are not very obvious. The flow stress constitutive equations of hot deformation for 42CrMo steel were developed. The predicted flow stress curves by the developed constitutive equations well agree with the experimental results, which confirmed that the proposed deformation constitutive equations can give an accurate and precise estimate of the flow stress for 42CrMo steel, and can be used for the analysis problem of metal-forming processes.