Plug-In Electric Vehicles (PEVs) have been developing to cope with fossil fuels pollution, which is produced by Internal Combustion Engine (ICE) vehicles in big cities. The vehicles are equipped by strong batteries, which can be charged and discharged for many times. This significant aspect enables vehicles to participate in various electricity power markets. In this paper, optimal sizing of parking lots along with Demand Response (DR) programs are presented to decline distribution network loss and voltage drop. Times of Use (TOU), Critical Peak Pricing (CPP) and Real Time Pricing (RTP) as three major time-based DR programs are applied to the problem. Genetic Algorithm (GA) is employed to solve the optimization problem. Simulation is carried out on a 33 bus radial distribution network. Results demonstrate optimal sizing of parking lots along with implemented DR programs has a significant effect on improving network loss and voltage drop.