We report a simple thermal treatment method for direct growth of InAsSb films on GaAs (001) substrates for the first time. The properties of the grown InAsSb films are systematically characterized by scanning electron microscopy, atomic force microscopy, X-ray diffraction, photo-luminescence and Hall measurement. It is found that the grown InAsSb films by this method have high quality with very smooth, mirror-like morphology, good electrical and optical properties. In particular, strong photoluminescence peak at around 3660 nm can be observed even at room temperature, which demonstrates the capabilities of the grown InAsSb films for room temperature MIR optoelectronic application. The mechanism for this growth method is discussed in details. We believe that this work provides a simple and feasible buffer-free strategy for the growth of high quality InAsSb films directly on GaAs substrate and it may also benefit other heteroepitaxial growth.