Atomic scale characterization of cleaved surfaces of cubic GaN (c-GaN) epilayers was established in real space. Using cross-sectional scanning tunneling microscopy (XSTM), c-GaN epilayers grown on GaAs (001) by low pressure MOVPE were investigated. The analysis of STM observations revealed that the atomic arrangement of the c-GaN (1 10) surface depended on the crystal quality of the c-GaN epilayer. For high quality c-GaN regions, the STM image confirmed that the c-GaN (1 10) surface was a 1 Â 1 reconstruction in analogy to GaAs. On the other hand, for poor quality c-GaN regions, a structure different from the 1 Â 1 reconstruction was observed, which was the disordered structure composed of atomic rows along the growth direction. In addition, stacking faults parallel to the c-GaN (111) plane, which gave rise to hexagonal GaN inclusions in the c-GaN epilayer, were characterized in real space.