The MBE growth of Si doped GaAs on slightly misoriented (111)A substrates was examined. The conduction types and the carrier concentrations were greatly affected by the degree of substrate misorientation. In the growth on an exactly (111)A oriented substrate, substantially all Si atoms acted as acceptors. With increase of the degree of misorientation and/or with increase of the flux ratio (JAs4 /JGa), the number of donor-site Si atoms increased, that is, the hole concentration first decreased by autocompensation of Si and then the conductivity reversed from p-type to n-type. The photoluminescence of highly compensated (111)A films showed characteristics of donor-acceptor pair recombination. A doping mechanism including preferential decomposition of As4 molecules at the steps was proposed.