Abstract Atomic resolution STM/STS has been performed on the cleaved surface (BiO layer) of Bi 2Sr 2CaCu 2O y at cryogenic temperatures down to 4.2K. The STM image taken at 4.2K clearly demonstrated atomic corrugations of Bi atoms, securing the vacuum tunneling in the present experiment. Based on the spectroscopic measurements, it was concluded that the BiO layer is essentially insulative with an energy gap. However, superconducting gap structure was often observed, possibly reflecting the electronic nature of CuO 2 layer. Furthermore, it was found that the barrier height is an important factor determining the shape of the tunneling spectrum.