Using double crystal x‐ray diffractometry (XRD) and transmission electron microscopy (TEM), the annealing effects on heavily carbon‐doped GaAs films were studied. From isochronal annealing, the evolution of compressive strain in carbon‐doped GaAs films was observed by XRD. From cross‐sectional TEM, unusual misfit dislocations with extra‐half planes on the GaAs side were observed in the sample annealed at 900 °C for 30 min in addition to normal misfit dislocations with extra‐half planes on the film side. A possible mechanism for the formation of such misfit dislocations is proposed based on the over‐relaxation of misfit strain in the film. ©1995 American Institute of Physics.