In recent years, the wireless local area networks (WLAN) access points (APs) are being deployed rapidly in the offices and the campuses to satisfy the quality of service (QoS) requirements such as network bandwidth. However, many empirical studies show that a large fraction of idle WLAN resources result in the significant energy losses. To reduce the energy consumption without violating the QoS requirement, we propose a QoS-aware AP energy saving mechanism using software defined network (SDN). We leverage the capability of the SDN controller to dynamically monitor the network condition information to estimate the network bandwidth requirement of the devices and then manage the network forwarding to provide seamless handover. Our experimental results show that the proposed scheme can effectively reduce the number of power-on WLAN APs while still providing QoS guarantee, such as network bandwidth and handover cost.