Effect of nitrogen addition on isothermal phase transformation behavior in a biomedical Co-Cr-Mo alloy without addition of carbon was investigated during aging at temperature between 973 K and 1273 K for up to 90 ks. In the Co-Cr-Mo alloy, phase transformation from fcc to hcp1 with super-saturation of Cr occurred by massive transformation. With prolonged aging treatment, a lamellar structure consisting of hcp2 with equilibrium of Cr and σ phases was formed at hcp1/hcp1 boundaries by a discontinuous/cellular reaction, expressed by the reaction equation hcp 1→hcp2+σ. The phase transformation is remarkably changed by addition of nitrogen. In the Co-Cr-Mo-N alloy, the phase transformation from fcc to hcp occurs with a significant long incubation period compared with that in Co-Cr-Mo alloy. Besides, lamellar M2N(M=Cr, Mo, Co) precipitates during phase transformation, suggesting that phase transformation of Co-Cr-Mo-N alloy may be dominant by the eutectoid transformation (fcc→hcp+M2N).