A linear gear shift mechanism for chainless vehicles includes a gear shift unit having a support rotator, transmission balls and driving posts, with the transmission balls disposed in the support rotator, with a cylindrical receiving portion disposed on each transmission ball radially, with the driving posts disposed in the cylindrical receiving portions along radial direction of the support rotator and rotating from radial direction thereof to but not reach axial direction of the support rotator; an axial power input rotator having an inward-tilted power input annular surface; an axial power output rotator having an inward-tilted power output annular surface, with the transmission balls clamped between inward-tilted power input and output annular surfaces and support rotator; a tread-required transverse power source meshing with axial power input rotator; an axial power transfer portion meshing with axial power output rotator axially; a transverse power output portion meshing with axial power transfer portion.