A novel switch-level fault simulation method is presented for MOS circuits that combines compiled switch-level simulation techniques with functional fault modeling. The simulator models both node stuck-at-zero, stuck-at-one faults and transistor stuck-on, stuck-open faults. During compilation the switch-level circuit components are compiled into functional models. The effect of transistor faults on the function of the circuit components is modeled by functional fault models that execute very fast during simulation. The differential fault simulation algorithm developed for gate-level circuits is adapted for use at the switch-level and is shown to perform well, although it incurs a higher overhead due to the dynamic memory properties of MOS circuits.