Using a single-symmetric relay feedback test, a method is proposed to identify all three parameters of a stable second-order plus time delay (SOPTD) model with equal time constants [k(p) exp(-Ds)=(tau s+1)(2)]. The conventional analysis of the relay auto-tune method gives 27% error in the calculation of k(w) for a large D/tau. In the present work, a method is proposed to improve the accuracy in the k(u) calculation by incorporating higher order harmonics. Three simulation examples are given for SOPTD and higher order time delay transfer function models. The estimated model parameters of the SOPTD model are compared with those of the Li et al. (1991) method and those of the Thyagarajan and Yu (2003) method. The open-loop performance of the identified model is compared with that of the actual system. The proposed method gives performance close to that of the actual system. Simulation results are also given for a nonlinear bioreactor system. The closed-loop performance of the model identified by the proposed method is close to that of the actual system.