Scanning microprobe reflection high-energy electron diffraction is a powerful technique to investigate the growth mechanism through microscopic surface observations during molecular-beam epitaxy (MBE) or metalorganic MBE. The Ga droplet formation and annihilation were observed in real time during GaAs growth with an alternative source supply. Droplets with a round shape whose diameter was the order of a micrometer were formed when only Ga was supplied over an amount of monolayer of GaAs on the surface. They were annihilated when As was supplied to grow a GaAs layer. At high substrate temperatures, the droplets on die vicinal GaAs surface moved upward to atomic steps before its annihilation. Anomalous features of droplet formation were observed during AlAs growth with an alternative source supply on the GaAs substrate. This is interpreted by the intermixing between Al and Ga atoms on the surface during growth. Indium droplets were also observed during InAs MBE growth at high temperatures, which were revealed to be the origin of die InAs hillocks.