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This paper presents the phase-locked loop (PLL) process control system designed by using the approach of linear quadratic regulator (LQR). By considering the loop filter (LF) of phase-locked loop is a part of the process to be controlled, an augmented system can be constructed when voltage control oscillators (VCO) is considered as an added integrator. The integral gain of the proposed augmented system obtained by LQR can be used as an optimal value to design the parameter of LF under the known value of natural frequency of the PLL. The experimental results in controlling the second-order lag pressure process using the proposed PLL show that the system response is fast without overshoot and steady-state error, the output disturbance effect can be rejected, the property of tracking can be obtained and the system can be controlled when process parameter is changed.