Variations of the electrostatic potential were investigated for oxide-passivated n-Si(111) surfaces with atomically flat terraces by measuring the force acting on an ultra-sharp tungsten probe that was attached to the quartz resonator of an atomic force microscope. When the probe-sample gap maintained a constant tunneling current, an enhancement of electrostatic force with a lateral extent of ̃5 nm was observed around underlying donor atoms and charged defects. Additional variations of the surface potential and the probe-sample capacitance across the surface steps were associated with excess electric charge at the step edge. © 2011 The Surface Science Society of Japan.