Energy efficiency and power factor were most essential performances for compressor unit driven by inverter-fed motors. The performances were simulated utilizing a parametric linear model under variable-frequency and variableconditions in this paper. The results showed that, at rated frequency, both the performances would fall down with lighter load; if frequency was reduced from rated, energy efficiency would fall even lower, though power factor rose up slightly. By comparison, at frequencies beyond rated, both the performances would go better. The results also showed, whether at infra- or ultra-frequencies, the motor overload capability was more restricted, so it could only be adopted in light-load conditions. This research could be helpful for optimization and energy saving of reciprocating compressor units running under variable frequencies and variable working conditions.