Organic-Inorganic perovskite solar cells (PSCs) have been attracted great attention because of their high power conversion efficiency (PCE), low production cost, and high flexibility. The most intensively studied light absorbing material is perovskite-structured CH3NH3PbI3 (MALI). Interestingly, it is reported that a small amount of chlorine incorporation into MALI increases charge carrier diffusion lengths (from 129 nm to 1069 nm), which enables planar structured PSCs with high PCEs. However, whether chlorine exists at the final perovskite film is under debate. Some studies report negligible amount or absence of chlorine in the final film, while others report detection of chlorine from the final film. In this study, we observed microstructure and chlorine content of Cl-incorporated MALI thin films with increasing temperature, using an in-situ nano-Auger spectroscopy and an in-situ scanning electron microscopy system. Precipitates begin to appear at the surface of Cl-incorporated MALI films, from lower temperatures compared to the MALI thin films. Moreover, grains of Cl-incorporated MALI films grow faster than those of MALI films. Local concentrations of chlorine at intragrain and the vicinity of grain boundary were analyzed to understand the microstructural evolution of the perovskite films.