Excessive voltage unbalance levels under normal operating conditions has become a power quality problem of concern in many power networks. Most available unbalance study only considers the three-phase three-wire power system and concentrates on the unbalance definitions, standards and effects. In this paper, a multiphase harmonic load flow program has been adopted to study the unbalance problem for both the three-phase three-wire system and three-phase four-wire system. The effects of various kinds of unbalance factors on the PCC voltage imbalance rate have been investigated, including the asymmetrical supply sources, un-transposed transmission lines and unbalanced three phase loads. The correlative relationship between these parameters and the PCC voltage unbalance rate have been set up. The results of this paper will help to enhance the understanding of the impacts of different unbalance factors on the PCC voltage unbalance rate.