Wireless micro sensor networks provide reliable monitoring of remote areas for data-gathering. Due to the limited battery capacity of sensor nodes, energy consumption plays an important role in the operation of WSN. This can be improved by using a protocol called Low Energy Adaptive Clustering Hierarchy (LEACH).Malicious attacks are generated in the network due to power supply, processing abilities and capacity for high power radio transmission. In this paper, one such attack namely BlackHole attack and its effect on single hop LEACH and multi-hop LEACH has been compared, simulated and analyzed.