The object of this paper is developing of a simulation model for performance evaluation of ai defense-gun system. We developed a three-step kill probability of areal target in case of one gun on one target considering : 1) estimating the target and warhead intercept point, 2) target vulnerability and 3) computing the kill probability. We used a Monte Carlo simulation method. This model can be used for probabilistic analysis giving results of sufficient accuracy with minimum requirement of input data. Also we developed a computer program according proposed algorithm and a set of experimental results using the proposed method are shown.