Abstract: A chainless transmission mechanism for a bicycle, including a right bottom bracket bearing axle holder and a left bottom bracket bearing axle holder bilaterally connected to the bottom bracket of a bicycle by connecting tubes and cone and locknuts to hold a bearing axle, a gear box coupled to one end of the freewheel hub of the bicycle, a transmission shaft having a bevel gear at one end engaged with a bevel gear in the bottom bracket bearing axle holder and a bevel gear at an opposite end engaged with a bevel gear on the freewheel hub, the bevel gear on the freewheel hub being mounted directly on the chain seat of the freewheel hub for gear clusters by a tightening up screw.