An automatic assembly machine includes a basis (10), a sliding device (30) horizontally movably attached on the basis for supporting a fan (40) thereon, a tube (27) fixed to the basis for receiving a plurality of bearings (28) therein, a diversion station for provisionally holding one bearing thereon, a mounting device (70) vertically movably attached to the basis, a horizontal driving device (60), a vertical driving device (80), and a control device (90) for controlling periodical movement of the horizontal and vertical driving devices. In one period, the vertical driving device drives the mounting device to catch the bearing from the diversion station and then the horizontal driving device drives the sliding device and the diversion station to move to allow the diversion station to receive another bearing from the tube and the mounting device to mount said bearing to the fan upon driven of the vertical driving device.