A folding mechanism for a treadmill including a handrail assembly pivotally attached by a coupling shaft to both sides of the front end of a base frame of a treadmill. Meanwhile, a positioning unit with an engaging element is provided such that the handrail assembly can be downwardly moved toward the base frame in an initial storage position. Moreover, an n-shaped foot rest is swivelably disposed at the center of the coupling shaft by a connecting arm. In this way, the n-shaped foot rest can be extended from a front crossbar of the base frame by the synchronic swiveling action of the coupling shaft when the handrail assembly is moved in the initial storage position. In match with an n-shaped supporting frame, the base frame can be brought into an upright position to achieve a vertical and stable folding state.