Organic-inorganic lead halide perovskites have drawbacks in applications due to insufficient and inadequate understanding of the materials. In particular, it is a priority to investigate CH3NH3PbI3 due to its suitable properties and its band gap energy of about 1.59 eV. In this work, we investigated the perovskite crystal with respect to structural, electronic, and optical properties because a single crystal is an ideal platform for examining essential characteristics. The spatial distribution of the perovskite structure was identified by transmission electron microscopy and Kelvin probe force microscopy. The built electronic structure is an important index for the carrier types depending on the surface potential distribution. In particular, a potential variation showed a mixture of the perovskite phase and a decomposed phase on the surface, which also implied the role of surface imperfections as initiation sites for degradation. Therefore, it is critical to verify the carrier types on the surface to enhance the possibilities of improvements for applications.