A base for a treadmill has a bottom board, two side frames and two stepping frames. The bottom board has two sides. The side frames are attached respectively to the sides of the bottom board, and each side-frame has a top. The stepping frames are connected respectively to the side frames, and each stepping frame has an anti-slipping frame and a side wing. The anti-slipping frame is mounted securely on the top of a corresponding side frame and has a top and an anti-slipping member mounted on the top of the anti-slipping frame. The side wing is connected to the anti-slipping frame and extends toward a side opposite to the bottom board. Accordingly, the base can fit with different running belts with different widths and is versatile and convenient in use.