Several algorithms have been proposed to provide better Quality of Service in the networks. Today, after developing the new architecture of networks that is Software Defined Networks, researches are trying to apply available features of traditional networks on this new network architecture. Hence, Quality of Service algorithms and mechanisms such as differentiated services, traffic engineering and admission control, that are available on traditional networks, are also implemented on these new networks. In addition, because of the programmability of these networks and the centralized control on network switches, the ability to implement many of these algorithms is provided. Based on what have been mentioned, in this thesis a new approach is presented to provide better Quality of Service in SDN networks using differentiated services mechanism. According to the proposed algorithm, this approach manages flows entering of the network and it assigns output port queues dynamically to every flow. If needed, it moves flows between queues so that while preventing increasing delays of existing flows it accepts a new flows by using available capacities of other queues. In this way, it increases the utilization of network resources. The importance of this work and its advantage will be shown in the obtained results. Keywords: SDN Networks, Differentiated Services (DiffServ), Quality of Service (QoS), Admission Control, Dynamic Queue Assignment, Dynamic Flow Management