Hydrogen implantation in GaAs is performed. Depth profiling of H is also done using 1.38 MeV He+ ion induced ERDA. Isochronal vacuum annealing in the large range of 100-700°C is then conducted to study hydrogen migration. For as-implanted sample, simulation gives a total hydrogen content of 5.6 × 1016 at./cm2, with the peak located 1000 angstrom from the surface. An interesting change in the profile is observed at 400°C. The concentration under tail reduces significantly and the surface peak becomes sharper compared with the as-implanted peak, but the total hydrogen content remains almost the same. A net loss of hydrogen, indicating hydrogen out-diffusion is seen above 400°C.