The sintering behaviour of AISI 4600(Ni-Mo) alloy steel powder for powder forging has been investigated. The powder compacts with the green density of 60-80%TD(theoretical density) were sintered at 1120℃ for 15 min in flowing Ar, N₂+3H₂ or H₂ atmosphere. The volume shrinkage and weight loss of powder compacts during sintering were larger in reducing atmospheres than in Ar atmosphere. In case of an addition of 0.45wt%C, the sintering kinetics was enhanced irrespective of the sintering atmosphere. In the Ar atmosphere, however the enhancement effect was the highest; this was attributed to the reduction of surface oxide and acceleration of Fe-atom diffusion by free carbon. The sintering behaviour of C-added powder compacts was also discussed, based on the chemical analysis of residual C and O content.