A novel method to characterize the mechanism of positive-feedback regeneration in a p-n-p-n structure during CMOS latchup transition is developed. It is based on the derived time-varying transient poles in large-signal base-emitter voltages of the lumped equivalent circuit of a p-n-p-n struc-ture. Through calculating the time-varying transient poles during CMOS latchup transition, it is found that there exists a transient pole to change from negative to positive and then this pole changes to negative again. A p-n-p-n structure, which has a stronger positive-feedback regeneration during turn-on transi-tion, will lead to a larger positive transient pole. The time when the positive transient pole occurs during CMOS latchup transition is the time when the positive-feedback regeneration starts. By this positive transient pole, the positive-feedback regenerative process of CMOS latchup can be quantitatively characterized.