[en] Tune modulation effect on nonlinear beam dynamics is studied. Different tune modulation conditions will lead to different motion properties. Boundaries on modulation condition are found to distinguish qualitatively different motions. The motion in resonance island with tune modulation is modeled by a driven pendulum. Its nonlinear property is analyzed, and investigated by simulation. Experiments have been done to observe the phenomena predicted by the theory. Many interesting results provide evidence for it. 12 refs., 39 figs., 4 tabs