The martensite transformation of Co−Cr−Mo alloys during cooling after holding at different austenitization temperatures (T_a), was investigated via high-temperature confocal laser scanning microscopy (HT-CLSM). The martensite starting temperature (M_s) increases with increasing T_a. Martensite area fraction (f_M) firstly decreases, then increases, and finally decreases with increasing T_a. Additionally, inverse martensite transformation, grain growth, and σ phase decomposition were examined during heating via differential scanning calorimetry and HT-CLSM. The effects of T_a on martensite transformation and phase composition of the alloys were analyzed. M_s and f_M were influenced by both grain size and pre-existing martensite at T_a of 900−950 °C, grain size alone at 950−1100 °C, and both grain size and elemental diffusion at 1100−1200 °C. This study will provide basic theoretical guidance for regulating the microstructure and enhancing the mechanical properties of Co−Cr−Mo alloys.