The linearity analysis of a passive mixer is presented. The distortion mechanism caused by switching operation of a MOS transistor is elucidated from the static and dynamic analysis of passive mixers. Furthermore, the maximum input and output level to keep linear operation and its required bias conditions are expressed by simple equations. The maximum linear output amplitude of the passive mixer is determined only by the local signal amplitude and it does not depend on input and output impedance. The calculated linearity performances agree well with simulated and measured results.