Heuristic algorithms are developed to partition an observable power system state estimation (PSSE) network into two or more observable subnetworks. The proposed heuristic algorithms partition a spanning tree of an observable PSSE network and guarantee the observability of all the partitioned subnetworks. These partitioning algorithms are based on using an integer-linear-programming eigenvector-based approach to have a good initial partition, and then on using an interchange method to obtain the optimal partition. The performance of these heuristic algorithms are evaluated by using several IEEE standard networks.