Active cancellation stealth is a significant developing direction in modern stealth technology field. In this paper, according to characteristics of linear frequency modulated (LFM) signal and nonlinear frequency modulated (NLFM) signal, the cancellation signal was designed. An important parameter called cancellaty which is used to measure the effect of cancellation is proposed. The basic theory of active cancellation stealth is introduced. Based on radar target fluctuation models, the formulas of the radar detection probability are given. Combining the definition of cancellaty with radar detection probability, the effective scope of the cancellaty is ensured. Simulation results show the effectiveness and the practicability of active cancellation.