The amount of carbon in GaAs and (Al,Ga)As films depends on the orientation of crystals grown by metalorganic chemical vapor deposition. We investigated the relation between the incorporation of carbon and the orientation of crystals using (100), (311)A, and (311)Bsubstrates. The concentration of electrons on (311)Bsubstrates of unintentionally doped films was higher than those of the films on (100) and (311)Asubstrates. The films grown on (311)Bsubstrates did not showp‐type behavior even when they were grown with a fairly low V/III ratio. The relative intensity of the free‐to‐carbon acceptor luminescence of the films grown on (311)Bsubstrates was smaller than that of films grown on the other substrates. This is consistent with the results of carbon contamination indicated by secondary ion mass spectra. Furthermore, a reduced peak in photoluminescence caused by defects was observed when (311)Bsubstrates were used.