Methylammonium lead halide perovskites have been developed as highly promising materials to fabricate efficient solar cells in the past few years. The real impact to energy applications relies on the understanding and controlling of the stability of the material. 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 a 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.