Phase synchrony among cortical brain regions is a key aspect of the working memory process. In this study we utilized phase synchrony analysis of EEG signals to construct brain networks and investigate the rest and working memory states of the brain. The maintenance period of working memory demonstrated higher global and local phase synchronization compared to the rest condition in the theta, alpha and beta bands. In contrast, during the manipulation period, we observed high synchrony between brain areas in the theta band and low phase synchrony in the alpha and beta bands. These alterations between different phases of working memory illustrated the critical behavior of brain networks and their highly dynamic characteristic.