InP thin crystalline films were grown on (100)-oriented GaAs by molecular beam epitaxy (MBE) and evaluated by RHEED, SEM and IMA. Thin films with high crystalline quality were obtained when the substrate temperature was about 240°C and temperatures of In and P cells were 840~880°C and 370~400°C, respectively. Furthermore, Sn atoms were easily doped into InP during MBE growth, and the surface morphology of InP was greatly improved by Sn doping.