A linear gear shift mechanism for chainless vehicles, comprising: a gear shift unit having a support rotator, a plurality of transmission balls and a plurality of driving posts, with the transmission balls spaced apart from each other and movably disposed on the support rotator, with a cylindrical receiving portion disposed on each said transmission ball along a radial direction thereof, wherein the driving posts rotate from the radial direction of the support rotator to but not reach an axial direction of the support rotator; characterized in that the driving posts have inward ends movably disposed in the cylindrical receiving portions, respectively, along a radial direction of the support rotator; an axial power input rotator having an inward-tilted power input annular surface and inputting power along the axial direction of the support rotator; an axial power output rotator having an inward-tilted power input annular surface and outputting power along the axial direction of the support rotator, wherein the transmission balls are movably clamped between the inward-tilted power input annular surface, the inward-tilted power input annular surface and the support rotator; a tread-required transverse power source for meshing with the axial power input rotator, wherein the tread-required transverse power source has a transverse power bevel gear, and the axial power input rotator has an axial power input bevel gear, such that the transverse power bevel gear meshes with the axial power input bevel gear; an axial power transfer portion for meshing with the axial power output rotator, wherein the axial power output rotator has an axial power output spur gear, and the axial power transfer portion has an axial power transfer spur gear, such that the axial power output spur gear meshes with the axial power transfer spur gear; and a transverse power output portion for meshing with the axial power transfer portion, wherein the transverse power output portion has a transverse power output bevel gear, and the axial power transfer portion has an axial power transfer bevel gear, such that the transverse power output bevel gear meshes with the axial power transfer bevel gear. a gear shift unit having a support rotator, a plurality of transmission balls and a plurality of driving posts, with the transmission balls spaced apart from each other and movably disposed on the support rotator, with a cylindrical receiving portion disposed on each said transmission ball along a radial direction thereof, wherein the driving posts rotate from the radial direction of the support rotator to but not reach an axial direction of the support rotator; characterized in that the driving posts have inward ends movably disposed in the cylindrical receiving portions, respectively, along a radial direction of the support rotator; an axial power input rotator having an inward-tilted power input annular surface and inputting power along the axial direction of the support rotator; an axial power output rotator having an inward-tilted power input annular surface and outputting power along the axial direction of the support rotator, wherein the transmission balls are movably clamped between the inward-tilted power input annular surface, the inward-tilted power input annular surface and the support rotator; a tread-required transverse power source for meshing with the axial power input rotator, wherein the tread-required transverse power source has a transverse power bevel gear, and the axial power input rotator has an axial power input bevel gear, such that the transverse power bevel gear meshes with the axial power input bevel gear; an axial power transfer portion for meshing with the axial power output rotator, wherein the axial power output rotator has an axial power output spur gear, and the axial power transfer portion has an axial power transfer spur gear, such that the axial power output spur gear meshes with the axial power transfer spur gear; and a transverse power output portion for meshing with the axial power transfer portion, wherein the transverse power output portion has a transverse power output bevel gear, and the axial power transfer portion has an axial power transfer bevel gear, such that the transverse power output bevel gear meshes with the axial power transfer bevel gear.