The effect of AsH3preflow on initial stages of GaAs grown on Si was studied by transmission electron microscopy. As the temperature of AsH3preflow was increased, the surface of the Si substrate became rougher: At 1000 °C, many threading dislocations originating at the rough surface were observed in the GaAs layer. In contrast, at low temperature, 450 °C, the Si surface became smooth and the dislocations were reduced. Furthermore, the Si surface was still smooth after long AsH3preflow at 450 °C. The roughness of the Si surface is attributed to the supply of As atoms to the Si surface at high temperature.