In this paper, in the presence of unmodeled dynamics, sliding mode control of the DC servo mechanism is analyzed via describing function techniques. In the presence of a second or higher order unmodeled dynamics, the limit cycle problem will occur in a conventional sliding mode controller with signum function. Hence, a fractional interpolation based smoothing scheme is proposed to eliminate the limit cycle, and meanwhile achieve the designated tracking precision bound with an adequate and yet moderate gain. A DC servo motor with unmodeled electrical and sensor filtering dynamics is used to show the validity of the analysis of the sliding control as well as the effectiveness of the proposed new fractional interpolation control scheme.