Abstract: By comparing the relationship of room temperature yield strength with the reduction of cold rolled MP159 alloy before and after aging at 650℃, and by means of TEM examination on the corresponding microstructures, the occurrence of fcc(α)→hcp(ε) martensitic transition in MP159 alloy during cold deformation and the variation of the platelet E phase during aging have been Studied. The results show that the platelet E phase is really formed when the cold deformation reaches a critical value, and both the amount and the width of the platelet B phase would further increase during 650℃ aging. The increases of the amount and the width of the platelet εphase result in an additional increase of yield strength. Therefore, it is concluded that the additional hardening effect of MP159 alloy after aging is not only from the precipitation of Ni3X disperse phase but also from the volume fraction increase of the platelet ε phase. Further occurrence of the martensitic transition during aging may be due to the elimination of residual compression stress within matrix induced by the fcc(α)→hcp(ε) transition during cold deformation