Abstract: A control mechanism of a bicycle gearshifting device is composed essentially of a body set provided with a hollow cylindrical portion over which a cable-locating wheel set and a rotator set are fitted. The cable-locating wheel set is provided with a fastening seat for fastening the cable head of a gearshifting cable, and a control arcuate segment for disposing the elastomer. The rotator set is provided with an arcuate protrusion for actuating the cable-locating wheel set to turn at the time when the bicycle handlebar is rotated, thereby activating the gearshifting cable and a derailleur of the bicycle.