Objectives: The objective of PMU placement in power system in a cost effective way to determine a minimum set of PMU’s so that the whole system is observable. Methods/Analysis: This paper presents a simple heuristic method. The principle is based on system topology that a PMU placed at a node should not create more than one unobservable islands. In order to use this principle, a condition, defined as tree break condition is used. Findings: The results obtained with this heuristic method are placement location of PMUs, observability of system with and without zero-injections under normal operating conditions and for line outage contingency. The systems used are typical 6, 7 bus systems and IEEE 14, 30, 57 bus systems. Novelty/Improvement: The results obtained by this method are simple, optimum and efficient than the methods used previously.