The analysis of distortion behavior in a CMOS passive mixer is presented. We use a simple device model with continuous equation to characterize the MOSFET switch in different operating regions. The nonlinear behaviors of passive mixers based on different switch topologies are investigated with power series analysis, which demonstrates that the CMOS switch based approach exhibits a better linearity performance due to the distortion cancellation of even order nonlinearity. The calculated conversion gain and linearity performances are compared with simulation results.