The influence of (cooperative) adaptive cruise control (CACC/ACC) on rear-end crashes have been conducted by some previous studies. However, they usually did not consider the degeneration from stable CACC to unstable ACC when vehicle-to-vehicle (V2V) communication is not available. California PATH program validated the instability of ACC system by using real experimental data, which might have negative impacts on safety. Therefore, this paper evaluated the impacts of degeneration of CACC vehicles on rear-end crash risks. A micro-simulation based method was adopted to evaluate rear-end crash risks with the increase of the CACC market penetration rate (MPR). The results obtained were quite different from those in previous studies and showed that the rear-end crash risks of the full MDV flow increase before CACC MPR increases to a certain level. It was found that the instability of ACC vehicles might be the main factor for the negative results. The results of this paper provide helpful insight into the management of CACC vehicular flow in the V2V communication environment.