A cushioning mechanism of a treadmill is interposed with an auxiliary plate between a base frame and the running board. The cushioning mechanism includes two hanging support pieces, two hanging swing pieces, and a flexible elastic element. When the running board is subject to a downward force and sinks, the flexible elastic element will be pulled and expanded externally. When the external force is eliminated, the running board will be returned back to its original position by the self-restoring ability of the flexible elastic element. In this way, an expected and optimal cushioning effect is achieved.