This paper presents a hybrid modified binary particle swarm optimization and fuzzy system for obtaining the optimal number of PMUs which makes the power system completely observable. Observability assessment is done by means of topological observability rules. Moreover a new rule is employed which can decrease the number of required PMUs for full network observability. In all of the investigations, the effect of zero-injection buses in the power system is considered. Developed PMU placement method is illustrated using IEEE standard systems to demonstrate the effectiveness of the proposed algorithm. The results show that the whole system can be observable with installing PMUs on less than 20% of system buses.