Optimum design for a window layer thickness of GaAlAs‐GaAs heteroface solar cell has been performed regarding the effect of reflection loss. It is shown that the GaAlAs layer thickness greatly affects the reflection characteristics of the cell and that there is an optimum thickness for the GaAlAs layer. Thus the GaAlAs layer thickness must be designed so as to minimize the reflection loss, as well as the absorption loss by the window. Optimum thickness for the GaAlAs layer is about 0.03 &mgr;m, and the power reflection loss for the cell with optimized double antireflection (AR) coatings is as low as 1.09 % for AM0 solar radiation.