A limit cycle test is discussed from the viewpoint of the plant identification, and a practical identification method using the modified limit cycle test is proposed. The limit cycle test is a convenient way to estimate some information of the plant dynamics but only few characteristics are obtained by the conventional way. In this work, the limit cycle test is modified so that the manipulated variable and the process value may contain fractional harmonics, and the plant model is estimated with those fractional harmonic components as well as the basic components. The use of the fractional components enables us to identify the plant model accurately in a noise environment and to handle more sophisticated model: a second order lag plus all-pass filter type one. An experimental example of a thermal plant is illustrated to show the effectiveness of the proposed method where an LQI controller is designed with the identified model of the plant.