Surface potential measurements of Si nano-structures by KFM (Kelvin probe force microscopy) are reviewed from a viewpoint of device research. For the development of sub-0.1 µm MOSFETs and future novel devices like dot-based single-electron tunneling transistors, it is most important to know the internal potential distribution of devices. In this review, first, the most fundamental structure of p-n junction formed on Si surface is described. It is noted that an unexpectedly small potential difference across the p-n junction was detected. The surface band bending due to the trapped charge would probably be responsible for almost flat profiles. Next, the potential distribution in the surface channel region of MOSFET-like structure on SOI (silicon-on-insulator) substrates is shown under several device-bias conditions. Finally, it is shown that Si quantum dot is recognized in KFM images after charge-injection process by generation of a small but significant potential difference at the dot site. These results indicate that KFM is undoubtedly useful in detecting potential distributions of device surfaces with or without device-driving voltage sources.