Abstract: Probability ideas and approaches are applied to the margin analysis of the robustness of control systems. The relationship between the perturbation range of the controlled plant and probability risk are studied. Comparison between the robustness margin in traditional sense and that in probabilistic sense is made in detail. Research results and examples have shown that on allowing the stability demands to be unfulfilled with a very meager probability, the robustness margin can often be magnified dramatically in the probabilistic sense. These results have important theoretical sense and reference value in the engineering practice.