Wireless Sensor Network (WSN) is being emerged as a prevailing technology in future due to its wide range of applications in military and civilian domains. These networks are easily prone to security attacks. Unattended installation of sensor nodes in the environment causes many security threats in the wireless sensor networks. There are many possible attacks on sensor network such as selective forwarding, jamming, sinkhole, wormhole, Sybil and hello flood attacks. Sinkhole attack is among the most destructive routing attacks for these networks. It may cause the intruder to lure all or most of the data flow that has to be captured at the base station.