The effect of vacuum annealing on the oxidation behavior of milled WC-15%Co powder mixture has been studied. A cobalt component in the milled powder mixture was oxidized preferentially above 175$^{circ}C$ in air. The specimens showed a steady increase in weight at 175$^{circ}C$ but did constant weight followed by rapid increase in specimen weight at the beginning above 20$0^{circ}C$. Oxidation of the milled powder mixture was significantly suppressed by vacuum annealing at 30$0^{circ}C$ for 10 h. Suppression of oxidation by vacuum annealing and different oxidation behaviors of the milled powder mixture between 175$^{circ}C$ and 20$0^{circ}C$, were attributed to removal of strain energy stored in the cobalt powder during vacuum annealing or oxidation treatment above 20$0^{circ}C$. The role of stored strain energy on oxidation of milled WC-15%Co powder mixture was proved by X-ray diffraction method and differential thermal analysis.