Scanning tunneling microscopy (STM) studies have been carried out to clarify the initial growth mechanisms of ultrathin Fe films on GaAs(001) substrates. For the submonolayer growth of Fe at room temperature (RT), the STM image showed many small clusters on the flat terraces of the c(4×4) reconstructed GaAs(001) surface. The typical size of the clusters seen in the STM image is about 15 Å which is nearly equal to the size of the basis structure of the c(4×4) reconstruction. It is also discussed that the formation of the clusters depends on the reconstruction of the GaAs(001) surface.