It is useful to understand the effect of the irradiance and temperature on the solar cell and module performance, in order to estimate their I-V curves under various climatic conditions. This paper studies the effect of the temperature on the silicon and GaAs photovoltaic cells. Also in this study, the irradiance effects on both Si and GaAs solar cells will be presented. The study shows that, the rise in V_oc, P_max,and maximum efficiency with temperature of GaAs solar cell degradation was approximately 1/4, 1/3, and 1/3 for Si photovoltaic cell respectively. Si solar cell degraded more than GaAs solar cell. Therefore, this study demonstrates that using GaAs photovoltaic cells instead of Si solar cell is preferred for space applications.