A hydraulic driving system for motorcycle gear shift includes a primary cylinder combination, a secondary cylinder and a set of delivery conduits for handling hydraulic fluid connected therebetween, in which two primary cylinders with different diameters are oppositely formed in a hollow portion of a motorcycle handle and the two primary cylinders are respectively fluidically communicated with the secondary cylinder to reciprocatively and linearly move a gear rack and to drive the shaft of the gear shift by rotating a twist bar, formed on the handle, having an inclined-plane tongue alternatively actuating the two primary cylinders in the handle.