A-plane GaN grown on r-plane sapphire is a promising material for optoelectronic applications such as laser diodes and LEDs. The absence of a built-in electrostatic field in the nonpolar a-plane GaN-based quantum wells can limit the piezoelectric polarization effect and result in higher emission efficiency. The knowledge of the planar anisotropic nature of nonpolar a-plane GaN is important to understand the growth mechanism of a-plane GaN is fundamental for optoelectronic device design. Two a-plane GaN samples were grown on r-plane sapphire, one being grown on intentionally miscut 4 degrees from r-plane sapphire. The films exhibited different surface morphologies and were characterized using imaging cathodoluminescence and SEM. The ordinary and extraordinary indices of a-plane GaN thin film were obtained using a prism-coupling technique. It was found that one sample the c-axis lay in the plane of the thin film as expected, and in the sample grown on miscut sapphire the c-axis was out of the plane of the film.