Wireless sensor networks (WSN) are widely developed to monitor different phenomena in a variety of areas including nature, medical centers, home automation, industrial and military applications. Such development in many different fields, raises important security issues related to the reliability of the WSNs. Due to the resource constrained nature of the WSNs, these networks are the target of many different types of attacks and prone to failure. In this paper, we consider the collision attack. An attempt has been made to measure the impact of the collision attack on the performance of WSNs under variety scenarios performed by the attackers. The main contribution of this paper is to present that although the attack does not consume much energy of the attacker, it can highly disrupt the normal operation of the target sensor networks. The implementation of the proposed attack model has been done by using NS2 network simulator.