Abstract: Frequency domain identification of non-integer order systems is a potent tool for modeling the non-integer systems.In this paper,a recursive least squares algorithm for frequency domain identification of non-integer order systems is presented.The algorithm is deduced in detail,and is validated with the known systems.The validation results show that the algorithm is the generalization of the the recursive least squares algorithm for integer order system identification,and not only the non-integer order systems but also the integer order systems can be identified with the presented algorithm.