Single ZnO nanowires are configured as field effect transistors and their electrical properties are characterized using scanning probe microscopy (SPM). Scanning surface potential microscopy is used to map the electric potential distribution on the nanowire. Potential drop along the nanowire and at the contacts are resolved, and contact resistances are estimated. Furthermore, conductive SPM tip is used as a local gate to manipulate the electrical property. The local change of electron density induced by a negatively biased tip significantly affects the current transport through the nanowire.