A sealing device for a gear box includes a box having a hole defined through a wall thereof and a shaft having a first end rotatably engaged with an inside of the box. A second end of the shaft is supported by a bearing which is received in a frame engaged with the hole of the box. A cap is connected to an end of the frame in the box and mounted to the second end of the shaft. A space is defined between a skirt and a protrusion of the cap so as to receive a seal therein. The seal has a flexible plate which is engaged with the end of the frame in the box. The flexible plate effectively prevents oil in the box from entering a gap between the bearing and the frame when the shaft is not rotated.