Increasing popularity of real-time applications has imposed the need to ensure quality of service for the applications in network.Therefore, providing quality of service in today’s networks has become an important goal. Considering the limitations of distributed control plane in traditional networks, in order to provide a solution to this problem SDN architecture has emerged which by separating the control plane from the data plane, creats a flexible platform for the implementation of any desired quality of service algorithm. In this thesis ,a new method in SDN architecture is presented to provide quality of service for delay-sensitive real time applications. In this method, the SDN controller monitors the status of the existing queues in the output ports of the switches in the network in consecutive periods of time ,then based on the collected information in case of inappropriate conditions in high priority queue some of the flows in this queue are transferred to lower priority queues. By doing this, delay for real time flows in the network can be controlled. The main features of this algorithm that distinguishes it from pervious works are the way the queue length in SDN switch i measured and the calculation of delay of queues using the queue length. Simulation results show the effectiveness of the proposed algorithm in guaranteeing required level of delay for delay-sensitive applications considering the capacity of queues on a selected path. Keywords : SDN, QoS, OpenFlow