The amorphous phase formation and initial crystalline reactions at Pt/GaAs interfaces has been investigated via high-resolution electron microscopy (HREM), microdiffraction, and energy dispersive spectroscopy (EDS). A thin amorphous intermixed layer consisting of three elements, platinum, gallium, and arsenic was observed at Pt/GaAs interface in an as-deposited sample. This interlayer grew to 4.5nm in an amorphous state upon low temperature(e.g. 200°C) annealing by a solid-state amorphization reaction. Following the growth of the amorphous interlayer, subsequently, Pt3Ga and PtAs2 phases nucleated within the amorphous layer and grew at the Pt and GaAs sides, respectively. We also observed the same reaction processes with in-situ annealing HRTEM.