Some relations between sensitivity performances and duplexer performances (losses, isolations between Tx and Rx terminals) are obtained. We simulate system noise figure levels from duplexer performances and low noise amplifier parameters by using a nonlinear NF model at the viewpoint of a system design. A nonlinear equivalent circuit model is proposed for analyzing distortion of SAW duplexer. Inter modulation distortion parameters are directly calculated by using the above nonlinear model. Consequently, desired duplexer performances to achieve sufficient system performances of WCDMA terminals can be determined by these procedures.