In this paper, a method deciding the permissible gains of the P and PT controllers for a retarded process under stable condition is proposed. For analysis, the controlled process is assumed to be first-order system with time delay. At first, the adjustable parameter sets causing stability limit are obtained based on the frequency domain condition which makes the roots of the characteristic equation locate on the imaginary axis. And the cut-in frequency ${omega}_p$ to bring the parameter set to P control from PI control is derived is derived in terms of L and $T_m$ then ${omega}_p$ is used to compute the maximum gain with stable condition. The results indicate that the permissible controller gains can be described by a unique if parameters L and $T_m$ are know.