Among electric vehicles, attentions are especially focused on plug-in electric vehicles of which battery can be charged from grid. Plug-in electric vehicles can cause some problems for power system such as peak load increasing. For reducing these bad effects, vehicle-to-grid (V2G) technology is suggested. V2G describes a system in which plug-in electric vehicles communicate with the power grid to sell demand response services by either delivering electricity into grid or by throttling their charging rate. V2G needs some infrastructures for implementation. The most important one is bidirectional power flow that has many problems like tripping of network protectors, and disruption of voltage control systems for realization. Due to the fact, in this paper a stage before V2G is introduced to utilize from potentially plug-in electric vehicles benefits. This concept uses from inherent network unidirectional power flow character to provide frequency regulation service. The details of this idea and results from its application to the electricity market are described.