The possibility of employing the demand response in the electricity market environment can lead to an improvement on the power system reliability and unit's production scheduling. To inquire these concepts, two scenarios are defined to schedule day-ahead energy and spinning reserve market simultaneously based on deterministic criteria. In the first scenario, reserve capacity can only be provided by committed generators while in the second scenario, the contribution of demand response in reserve market is permitted. This approach has been implemented over the IEEE 24-bus test system using mixed integer linear programming (MILP) and solved by GAMS software. The achieved results are compared from both economic and technical point of views which indicate the advantages and disadvantages of applying demand response in the system scheduling. Moreover, two different bidding scenarios for demand response are examined.