Routers in any infrastructure are most interesting part for any attacker because almost whole traffic flows through these and drastic activities can be done with routers. Packet drop attack is one of the attacks that happen on router by dropping out the data packets flowing. Two flavors: Black and Gray hole creates critical situations as Black hole drop out whole traffic whereas Gray hole drops selective data packets which makes it more difficult to detect. In this paper, we proposed an algorithm to detect these attacks by using the Flow conservation property of any network device. Work has been done with the help of packet count on the interfaces of the router.