Methylammonium lead halide perovskites as highly promising photovoltaic materials have been found unstable in air. We investigated the degradation of CH3NH3PbI3 by air exposure using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The stoichiometric sample was grown with co-evaporation of PbI2 and CH3NH3I on an Au coated Si wafer. It was found that the perovskite thin film gradually turned to PbI2 in air, accompanied with complete removal of N and substantial reduction of I. It was also observed that PbI2 crystallization roughened the film and resulted in a partial exposure of the Au substrate.