It is very important to understand the electronic structure of GaAs and GaSb for fabrication of electronic and optoelectronic devices. Recently, tenary compounds, GaAs_1-xSb_x began to attract much attention from researchers because they were synthesized and expected to be new materials for optoeletronic devices. For x = 1 and x = 0, there have been a lot of computational studies to investigate their structural, electronic and optical properties, but few studies for general cases. Moreover, it has been known that “regular” density functional calculations does not describe the correct electronic structures including underestimated band gap and incorrect band splitting. To resolve such issues, we consider spin-orbit coupling effects and GW corrections on GaAs_1-xSb_x with the composition of x = 0, 0.25, 0.5, 0.75, and 1. To calculate their band structures, we make use of maximally localized Wannier functions to interpolate GW quasiparticle band structures. Based on our GW calculations, we discuss the trend of band gaps at various high symmetric points in the Brillouin zone with the composition x.