In this paper an adaptive control system with a periodic disturbance external model is considered for plants which are not necessarily of minimum phase. An external disturbance model which sets up outside the feedback loop is used to reject the external disturbance. As the parameters of disturbance model and plant model are not known, the adaptive control technique is applied. In this design one estimator is adopted for both plant model and disturbance model estimation. Stability analysis is provided, supported by a numerical simulation example