Microscopic distribution of growth rates on mesa‐etched GaAs(001) wafers was measured in real time during molecular beam epitaxy growth by scanning microprobe reflection high‐energy electron diffraction. It has been observed that the growth rate on the GaAs(001) surface near the edge of (111)Asurfaces becomes larger. The exponential variation of the growth rate as a function of the distance from the edge reflects surface diffusion of Ga atoms. The diffusion length on the (001) surface is estimated to be about 1 &mgr;m at 560 °C. The relatively larger diffusion length suggests that the incorporation rate of migrating Ga atoms by steps is much smaller than unity.