In this paper, we collect all available times of the light minimum of AR Lac from the literature and databases. Through an analysis of the O C curve by a weighted least-squares method, we found that the orbital period of the system shows a cyclic oscillation with a period of 50.93 yr superimposed on a long-term secular period decrease at a rate of dP =dt = (2.128˙0.060) 10 9 d=d. Since both components of AR Lac are late-type stars, and show strong Ca H and K emission lines, the orbital period variation may be caused by the magnetic activity of the system.