We have investigated the transition pressure, p t , of bulk GaAs from the zinc blende (B3) to the rocksalt (B1) structure using the local density approximation (LDA), generalized gradient approximation (PBE-GGA) and diffusion Monte Carlo (DMC). We took into account finite temperature effects (zero-point vibrational effects) as well as finite size corrections. Our DMC calculation using GGA trial nodal surface supports the higher value of the transition pressure (∼ 17 GPa) than the lower value of (∼ 12 GPa), both of which are experimentally reported values. This projection increases the transition pressure, p t , from DFT predictions, being of the same tendency as that for Si bulk crystal. The choice of the XC functional in DFT was found to significantly determine the phase transition pressure, while DMC gave more accurate results for this transition pressure.