We investigate the structural and optical properties of InGaN-based multi-quantum wells grown on a–c plane sapphire substrates by atomic force microscopy, high-resolution x-ray diffraction and temperature-dependent photoluminescence measurements. The multi-quantum wells grown on a-plane sapphire substrate show stronger exciton localization effect compared with that grown on the c-plane substrate, although the threading dislocation densities in the two samples are almost the same. We attribute the results to the different in-plane strain of quantum well grown on different substrate orientations at the same growth temperature. Since the exciton localization effect plays a key role to obtain high efficiency InGaN-based optoelectronics devices, the presented result should be emphasized.