This paper is concerned with derivation and tuning of PID control law. In the first half, output tracking for general nonlinear system is investigated by means of direct gradient descent control (DGDC), in which control input is manipulated directly so as to decrease a performance function based on gradients of the performance function and the steepest descent method. In the second half, PID control law for linear systems is derived by using the output tracking DGDC, and at the same time best tuning of PID controller parameters is established. The effectiveness of the proposed method was confirmed by computer simulations, where very good performance was observed with various examples.