Abstract: In this paper, application of PL mapping to characterize SI-GaAs crystal has been studied in detail. The experiment results show that PL mapping and its intensity of SI-GaAs wafer plays an important role in determining the performance of device. When selecting high quality SI-GaAs crystal for fabricating device, PL mapping homogeneity is an important parameter besides resistivity, mobilty, EPD, carbon concentration, EL2 concentration and their homogeneity, wafer surface quality.