In this paper, a simple and efficient PID auto-tuner is proposed. Firstly, a second order plus dead time model is identified based on two continuous relay feedback experiments. With the model available, the PID controller is designed based on sensitivity specification. The PID auto-tuner achieves a high performance for processes with various dynamics, including those with low- and high-order, small and large dead time, and monotonous and oscillatory responses. Simulation examples show the effectiveness and flexibility of the PTD auto-tuner.