The transition phase of GaAs from the zincblende (ZB) structure to the rocksalt (RS) structure is investigated by ab initio plane-wave pseudopotential density functional theory method,and the thermodynamic properties of the ZB and RS structures are obtained through the quasi-harmonic Debye model.It is found that the transition from the ZB structure to the RS structure occurs at the pressure of about 16.3 GPa,this fact is well consistent with the experimental data and other theoretical results.The dependences of the relative volume v/Vo on the pressure P , the Debye temperature θ and specific heat Cv on the pressure P,as well as the specific heat Cv on the temperature T are also obtained successfully.