With the development of restructured power systems and increase of prices in some hours of day and increase fuel price, Demand Response programs were noticed more by customers. Demand Response consists of a series of activities that governments or utilities design to change the amount or time of electric energy consumption, to achieve better social welfare or some times for maximizing the benefits of utilities or consumers. In this paper the effect of emergency Demand Response program on composite system reliability of a deregulated power system is evaluated using an economic load model, AC power-flow-based load curtailment cost function and reliability evaluation techniques. In this paper for calculation the reliability indexes, the Emergency Demand Response Program (EDRP) cost is considered and in each contingency state, the EDRP cost with the customer load curtailment cost is compared and the load appropriate value is selected for load shedding or participating in EDRP. In the next stage, the system and nodal reliability indexes are calculated. To investigate the impact of EDRP activity on composite reliability of restructured power systems the IEEE 6 bus Roy Billinton Test System is utilized. According to obtained results, EDRP using lead to increasing nodal and system reliability. It can be said that solving problems such as congestion in transmission lines, power system reliability decrease at load network peak hours, is impossible without customer interfering in power market. In other hand Consumer participation, makes the power markets more competition and enhance its performance.