The control problem of networked control systems via output feedback is considered in this paper. In the system, a sensor is connected to a linear controller/actuator via a network. The plant is controlled alternately by an open loop controller and a closed loop controller. Necessary and sufficient conditions for the exponential stability of such systems are derived in terms of the network dwell times and the system parameters. Based on the results, a simple procedure for designing the output feedback controller is presented. Numerical simulation shows the feasibility and efficiency of the proposed method.