Abstract: A gear selector mechanism includes a positioning disk fixedly mounted around one distal end of the handlebar of a bicycle. A plurality of teeth are each formed on a periphery of positioning disk and each has a guiding face and a stop face. An actuating member is rotatably mounted around the one distal end of the handlebar and includes an actuating disk rotatably mounted to the positioning disk. An actuating block is formed on the actuating disk to rotate therewith and has an arcuate pushing perimeter slidably rested on a fixing stub of the derailleur cable of the bicycle. A retaining block is formed on the actuating disk. A catch member includes a flexible arcuate body slidably in the positioning disk and having a lug securely retained in a space of the retaining block and having a click detachably engaged with each of the plurality of teeth.