Vehicular ad hoc network (VANET) is a new type of ad hoc network with the characteristics of highly dynamic topology, variable vehicle velocity and den- sity. In the performance evaluation of VANETs, tradi- tional collision probability model is not suitable because it poorly re°ects these characteristics. This paper focuses on the analysis of collision probability, which is a vital in- gredient for any performance evaluation in IEEE 802.11 system. On the basis of collision probability in WLANs, a new model is proposed to estimate collision probability in VANETs, which integrates the characteristics of VANETs. The model shows that the collision probability in VANETs is no longer a constant value as in traditional model, but a function of the factors re°ecting the characteristics of VANETs. Simulation results using Network Simulator 2 (ns-2) show the validity and accuracy of the proposed model.