Abstract: Based on the basic principle of semiconductor physics, an analytical theoretical model was proposed to show the relationship between the microwave pulse power threshold triggering the internal transient latch-up in the CMOS inverter and pulse width quantitatively by solving the continuity equations for minority carriers. By comparing the proposed theoretical model with simulated results and experimental data reported in literatures, the correctness of the theoretical model was verified. Furthermore, the model suggests that the power threshold triggering the internal transient latch-up in the CMOS inverter is a decreasing function of pulse width as the pulse width is short. However, there is an inflexion range and the power threshold varies little when the pulse width exceeds the inflexion range.