A linear gear shift mechanism, comprising: a support rotator; a plurality of transmission balls spaced apart from each other and movably disposed on a lateral annular surface of the support rotator, wherein one of a cylindrical channel and a cylindrical recess is disposed on each said transmission ball along a radial direction thereof; a plurality of driving posts with inward ends movably penetrating the cylindrical channels along a radial direction of the support rotator, respectively, or movably disposed in the cylindrical recesses along the radial direction of the support rotator, respectively; a gear shift unit movably connected to inward ends and outward ends of the driving posts when the inward ends of the driving posts movably penetrate the cylindrical channels, respectively, and movably connected to the outward ends of the driving posts when the inward ends of the driving posts are movably disposed in the cylindrical recesses, respectively, wherein the gear shift unit drives the driving posts to rotate from the radial direction of the support rotator to but not reach an axial direction of the support rotator; an axial power input rotator having an inward-tilted power input annular surface; and an axial power output rotator having an inward-tilted power output annular surface, characterized in that the axial power input rotator and the axial power output rotator are disposed on a same side of the transmission balls, whereas the support rotator is positioned beside the transmission balls in a manner to be opposite to the axial power input rotator and the axial power output rotator, so as to movably clamp the transmission balls between the inward-tilted power input annular surface, the inward-tilted power output annular surface and the lateral annular surface of the support rotator. a support rotator; a plurality of transmission balls spaced apart from each other and movably disposed on a lateral annular surface of the support rotator, wherein one of a cylindrical channel and a cylindrical recess is disposed on each said transmission ball along a radial direction thereof; a plurality of driving posts with inward ends movably penetrating the cylindrical channels along a radial direction of the support rotator, respectively, or movably disposed in the cylindrical recesses along the radial direction of the support rotator, respectively; a gear shift unit movably connected to inward ends and outward ends of the driving posts when the inward ends of the driving posts movably penetrate the cylindrical channels, respectively, and movably connected to the outward ends of the driving posts when the inward ends of the driving posts are movably disposed in the cylindrical recesses, respectively, wherein the gear shift unit drives the driving posts to rotate from the radial direction of the support rotator to but not reach an axial direction of the support rotator; an axial power input rotator having an inward-tilted power input annular surface; and an axial power output rotator having an inward-tilted power output annular surface, characterized in that the axial power input rotator and the axial power output rotator are disposed on a same side of the transmission balls, whereas the support rotator is positioned beside the transmission balls in a manner to be opposite to the axial power input rotator and the axial power output rotator, so as to movably clamp the transmission balls between the inward-tilted power input annular surface, the inward-tilted power output annular surface and the lateral annular surface of the support rotator.