Elementary growth processes involved in MBE of GaAs is discussed basing on the experiments done by a microprobe RHEED/SEM MBE system. The incorporation diffusion length of Ga, lambda(inc) during MBE growth was measured in situ on (001) and it was shown that lambda (inc) depends on the arsenic pressure in different manner for the different ranges of arsenic pressure, which suggests different order of arsenic reaction is involved in the growth process. The direction of intersurface diffusion between two facets is studied and it is concluded that the key parameter which control the direction is the incorporation lifetime, tau(inc) of Ga. Its arsenic pressure and the surface reconstruction dependencies an found as the major factors which determine the direction of the intersurface diffusion.