A numerical study of the effectiveness of phase change material (PCM) used on FORMOSAT-7 at the preliminary design phase is presented in this study. N-eicosane is used as the PCM for its melting temperature. To compare the performance of PCM, different messes of PCM are applied for high-power-dissipating component with short-duty-cycle. The results show that PCM can improve the thermal stability of component by not only moderating peak temperature for worst hot case but also preventing sudden temperature decrease when the power mode of component changed. However, mass addition of PCM reduces the duration of the maximum temperature and the minimum temperature due to the better thermal conductivity of solid phase. Therefore, an optimization of mass is suggested for the application of PCM.