Molecular dynamics simulation was carried out to estimate the diffusion coefficient and the mechanism of diffusion with a constant pressure and temperature algorithm. The following results were obtained: (i) there is little diffusion of a vacancy in solid GaAs, even near the melting point, (ii) the diffusion coefficient of an interstitial arsenic atom is larger than that of an interstitial gallium atom, (iii) after formation of a dumbbell pair in the [110] direction, an interstitial arsenic atom migrates in the [110] direction, while an interstitial gallium atom migrates in either the [110] or [-110] direction, and (iv) an interstitial arsenic atom pushes out a substitutional arsenic atom and then diffuses by an interstitialcy mechanism, while an interstitial gallium atom pushes out either a substitutional arsenic or gallium atom. Finally, all of migrating interstitial atoms in solid GaAs switch to interstitial arsenic atoms at the final stage of diffusion. (C) 2003 The Electrochemical Society.