Effect of phase transformation and grain-size variation of hot-pressed cobalt on its dry sliding wear was investigated. The sintered cobalt specimens were heat treated under different conditions and their tribological characteristics were examined. The sliding wear test was carried out against glass (83% SiO2) beads at 100N load using a pin-on-disk wear tester. A constant sliding speed of 0.38m/s and sliding distance of 600m were adapted. Worn surfaces, cross sections, and wear debris were examined by a scanning electron microscopy (SEM). X-ray diffraction (XRD) was utilized to identify phases of the specimen and wear debris. The cobalt specimens exhibited low friction coefficients of around 0.2. Thermal transformation of the cobalt from the hcp ε phase to the γ (fcc) phase during the wear was detected, which was deduced as a wear mechanism of the sintered cobalt. Typical wear characteristics of the cobalt including a thin detaching surface layer and fine wear debris were explained by the transformation. A correlationship between the grain size and the transformation was also explored.