Abstract: This paper discusses relationship between digital PID regulator and optimal feed back control for quadratic performance index with regard to discrete time linear time-invariant systems,presenting a method of optimally adjusting PID regulator parameters.As an ex醡ple,we consider the temperature control system(2 input/2 output linear time-invariant system) of creep testing furnace.Digital simulation and field test demonstrates that the method presented in this paper is useful and convenient,under steady-state conditions.