The phase transformation temperature, microstructure of 10%Co-WC cemented carbide, pure Co powder were analyzed using in situ X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope(TEM) in the paper. The results showed that, shearing martensite phase transformation was the main type in Co, the crystal orientation relationship was {111}_α//{0002}_β,<110>_α//<110>_β. and lamellar structure was formed as the stacking fault induced at the {0002} crystal plane during phase transformation. Compared with powder, the A_s and A_f were up about 80℃,the supercooling degree was increased as the slipping drag strengthened as the W and C atoms solid solution and precipitation.