After a main blackout occurs in a large power system, partitioning the power system into several subsystems which are restored in parallel can accelerate the restoration procedure. In this paper, a system partitioning method based on communities finding theory of complex network is proposed for black-start restoration, using similarity of the node voltages as the partitioning criterion. In the proposed method, by determining the voltage thresholds for network partitioning according to the black-start units and their node voltages from power flow calculation, the power system can be partitioned into several subsystems restored following the strategy of stepping up from zero. The obtained subsystems satisfy the partitioning requirements, that is, the nodes in a same subsystem are tightly inter-linked, the nodes in different subsystems are sparsely inter-linked, and all the subsystems have similar sizes. The effectiveness of the proposed method is validated by an application example on the IEEE 118-bus power system.