Scanning tunneling microscopy (STM) studies were carried out to clarify the initial growth mechanisms of ultrathin Fe films on GaAs(001) substrates. Observation revealed that Fe atoms initially form many small clusters on the flat terraces of the c(4 × 4) reconstructed GaAs(001) surface at room temperature. The typical size of the Fe clusters is about 1.5 nrn, which is nearly equal to the size of the basis structure of the c(4 × 4) reconstruction. It is also shown that the formation of the Fe clusters depends on the reconstruction of the GaAs(001) surface and the temperature of thermal treatment.