A first-principles plane wave method with the ultrasoft pseudopotential scheme in the frame of the generalized gradient approximation (GGA) is performed to calculate the lattice parameters, the bulk modulus Bo and its pressure derivative B＇0 of the hexagonal wurtzite GaAs (w-GaAs) by the Cambridge serial total energy package (CASTEP). Our calculations show that the most stable structure of the w-GaAs corresponds to the axial ratio c/a = 1.651 and the inteal parameter u = 0.374, consistent with other theoretical results. Also, the thermodynamic properties of the w-GaAs are investigated from the quasi-harmonic Debye model. The dependences of the normalized lattice parameters a/a0, c/c0, the axial ratio c/a, the normalized volume V/V0, the heat capacity Cv and the thermal expansion α on pressure P and temperature T are also obtained successfully.