Collision Mitigation Brake (CMB) is a part of the Pre-crash safety system intended to mitigate damage during vehicle collisions. In this study, the effects of CMB system on vehicle rear-end collision accidents were investigated. To this end, a dynamic vehicle simulation model was constructed, in which the sensor and the CMB model were built. By using this model, the reduction of collision speed by the CMB system was evaluated numerically. Here, the road surface friction coefficient is considered to have a large influence on the CMB. In addition, the Time to Collision (TTC) which is the judgment line of the brake initiation is the key factor to the efficient operation of CMB. Therefore, simulations were carried out on road surfaces with different friction coefficient and CMB system with different values of TTC. Generally, the effect of CMB is expected to increase with the rise of friction coefficient of the road and the value of TTC. A larger value of TTC will be preferred for the cases of roads with low friction coefficients. However, collision avoidances might occur when the large value of TTC is employed in low speed collisions. Finally, the findings revealed that the value of TTC should be controlled in such that the value is increased for roads with low friction coefficient and decreased for low speed collisions.