This paper presents a unified RF behavior model that simultaneously predicts the effects of noise, nonlinearity, impedance matching and frequency response and that enables efficient and accurate system simulation. The proposed modeling approach allows characterizing the RF effects incrementally to reduce iteration. A Verilog-A behavior model for an ultra-wideband CMOS low noise amplifier is developed for fast and accurate system simulation. The system simulation results show that the behavior model agrees well with the transistorlevel circuit with RMS error less than 0.79%. Ultimately, 87% reduction of simulation time is achieved.