Electronic properties of the hydrogen-terminated (H-terminated) silicon (Si) surfaces were characterized and optimized using an ultrahigh-vacuum (UHV) contactless capacitance-voltage (C-V) measurement system recently developed by our group. Unoptimized H-terminated Si surfaces were found to possess fast surface states having a discrete peak at 0.6-0.7 eV from valence band edge, and show surface states causing hysteresis in C-V curves. By a sufficiently long NH4F treatment followed by UHV anneal at 200 degreesC, a UHV-compatible completely passivated surface with no appreciable discrete states and no hysteresis could be obtained. Such a surface was thermally stable up to 250 degreesC in UHV. The most likely origin of the discrete fast surface states is Si-dangling bond, and that of the slow states is excess hydrogen.