Heteroepitaxy of ZnSe on GaAs As-rich (0 0 1) surface is investigated by Monte Carlo growth simulation using the atomistic model, which consists of interatomic bonding energies and takes into account both the surface-atom dimerization and the electron transfer between surface dangling bonds, donor bonds such as Ga-Se and acceptor bonds such as Zn-As. It is shown that, due to the heterovalency between ZnSe and GaAs, antisites and vacant defect structures are produced at the interface and these defects are suppressed when Zn atoms are first supplied on the GaAs surface. When Se atoms are first supplied on the substrate, the Se adsorption promotes As evaporation and the remarkable surface roughening occurs. These results are in qualitative agreement with recent experimental observations.