Due to the construction and network infrastructure of wireless sensor network (WSN) are known to be vulnerable to variety of attacks. In order to ensure its functionality especially in malicious environments, security mechanisms are essential. Several works have been done to secure WSN, but identification of insider attacker has not been given much attention. In the WSN system the malicious node behavior is different from the neighbor nodes. Instead of relying the untrustworthy neighbor node we use Dempster-Shafer theory (DST) of combined evidence to identify the insider attacker in WSN. This theory reflects with the uncertain event or uncertainty as well as uncertainty of the observation. The mathematical calculation shows the DST capability of identifying the insider attacker.