A collapsible mechanism for a treadmill is composed of a support frame, a stand, and an operational console. The stand includes a frame body, a first positioning assembly, and a second positioning assembly. The frame body has a bottom side pivotably attached to the support frame in such a way that the frame body is collapsible relative to the frame. The first and second positioning assemblies are connected with the frame body and the support frame separately. Each of the first and second positioning assemblies includes a first (second) fastener and a first (second) sheet member. The first and second fasteners are inserted through the first and second sheet members separately. Each of the first and second sheet members includes two first specific positions. The operational console is pivotably attached to a top side of the frame body and collapsible relative to the stand.