We have observed GaAs growth processes on an InP (001) surface thermally cleaned in an arsenic flux using a multichamber ultrahigh-vacuum scanning tunneling microscope (UHV-STM) equipped with a molecular beam epitaxy (MBE) facility. When either 1 monolayer (ML) or 1.5 ML GaAs was deposited on an InP (001) surface, two-dimensional growth was observed. On the other hand, island formation was observed for 2.5 ML GaAs deposition.