As a sequel to the previous works of Huang and Lee (Huang et al., 1998, 2001; Lee and Huang, 2001), this work presents a new on-line method for identifying block-oriented models of Hammerstein-type or Wiener-type nonlinear processes from a relay feedback experiment. The identification of linear subsystem is focused. A simple transfer function of linear subsystem is determined by using an adjustable P1 controller to develop a loop transfer function that has the dynamics of an integrator-plus-dead-time (IPDT) process. The static nonlinearity is represented by an invertible. Finally, the controller design using the identified model is illustrated.