The martensite transformation of Co-29Cr-6Mo-xCu (Co-xCu,x =0,1.5,2,4,wt％) alloys was investi-gated in order to explore ductile Co-based alloy without Ni for biomedical application.The effect of Cu on martensite transformation of Co-29Cr-6Mo alloy was investigated using scanning electron microscopy(SEM),transmission electron microscopy (TEM),X-Ray diffraction (XRD) and electron backscatter diffrac-tion (EBSD) analysis.EBSD results revealed that the fraction of γ-phase was increasing with increasing of copper content.In detail,the fraction of γ-phase of aged Co-1.5Cu alloy was 4.72％ and no γ-phase was found in aged Co-0Cu alloy.Furthermore,the fraction of γ-phase of aged Co-2Cu alloy and Co-4Cu alloy was 51.68％ and 46.17％,respectively.This demonstrated that the minimum Cu content which could inhibit martensite transformation in this alloy system was 2 wt％.Calculated phase diagram revealed that increasing the copper content of this alloy system could enlarge γ-phase zone,which means Cu could inhibit the formation of ε-phase and consequently stabilize γ-phase.Randomly distributed Cu-rich phase was identified in aged Co-4Cu alloy.The possible reason for the inhibition of martensite transformation by Cu addition is that alloying with Cu atom might decrease charge accumulations and increase atomic bonds,which increase the stacking fault energy ultimately.