The present application discloses an apparatus for mounting an O-ring to a work piece, which includes a body including a plurality of connecting holes for connecting a pressure source to the body; an annular groove formed on the body and adapted to receive the O-ring; and a plurality of air channels formed in the body, the plurality of air channels being in fluid communication with the annular groove and the plurality of connecting holes to provide negative pressure with the pressure source in the annular groove to suck the O-ring in the annular groove, the annular groove is coaxial with a groove formed on a surface of the work piece during the mounting so that the O-ring is dropped into the groove when the pressure source is disconnected from the body or provides positive pressure to the plurality of air channels.