CsCl films deposited on the MgO(001) surface were investigated using coaxial impact collision atom scattering spectroscopy which has been developed in the present study, in addition to reflection high-energy electron diffraction and atomic force microscopy. In the initial stage of the growth, the coexistence of α- and β-CsCl phase with their (001) parallel to the surface has been found. The orientation relationship of each phase was determined. By contrast, the thick film in the range between 100 and 200 9 consisted of α-CsCl with its (110) parallel to the surface. The structural transition from the former to the latter structure was observed to occur on annealing of the overlayer of 10-30 9 at 673K for 10min. The CsCl growth on MgO at room temperature is thought to occur in the Volmer-Weber mode.In the present investigation, the problem of the charging effect was successfully avoided. CAICASS has been shown to be a powerful technique for the structure analysis of insulator surfaces.