The linearity of the CMOS is of major concern in the design of many analog circuits. In this paper, the nonlinearity behavior of CMOS analog integrated circuits is investigated. The basic building block of analog integrated circuits such as differential amplifier with current mirror load has been chosen for harmonic distortion analysis. A mechanism to analyze the distortion of CMOS circuits in deep submicron technology that can be easily used to detect the distortion is built. The MOSFET model used for simulation is TSMC BSIM3 SPICE model from 0.13-um CMOS process technology. HSPICE circuit simulator tool is used for distortion analysis of CMOS circuits.