in this paper, input/output linearization (IOL) method using time delay control (TDC) and time delay observer (TDO) is presented. This method enables the IOL method to be applied to plants even when all the states of plant are not measurable or the measured plant output is very noisy. The designed control system requires neither an accurate plant model nor the realtime computation of plant nonlinearity. Consequently, the proposed control algorithm turned out to be computationally efficient and easy to design for nonlinear plants. In a simulation for a second order nonlinear plant, the output followed desired response well and the control performance appeared to be superior to IOL using TDC and numerical differentiation. Finally, in an experiment with a pneumatic servo system, we obtained results consistent with those from the simulation, and it was confirmed that the proposed control algorithm can be effectively used in a real closed-loop system.