Transmission congestion is one of the key elements affecting spike price and local market power. Reducing the demand and increasing the generation in distribution network may lead to relive the congestion of a transmission line. Plug-in hybrid electrical vehicle (PHEVs) parking lots and emergency demand response programmes (EDRP) are considered as two main options to manage the demand of a transmission bus. In this paper, a novel approach is proposed to determine the optimal capacity of EDRP and PHEV parking lots in order to relieve transmission congestion in a least-cost manner. The behaviour of EDRP participants and available capacity of the parking lot are mathematically modelled. Genetic algorithm (GA) with binary variables is employed to solve the optimisation problem. Simulation is carried out on a 14-bus transmission network. Results demonstrate the optimal contribution of parking lots and EDRP has a significant effect on mitigating congestion and decreasing the cost of the system.