A rear suspension system includes a bottom bracket integrally connected between two respective first ends of two chain stays and the bottom bracket is engaged with a recess defined between two lugs on the conjunction end of the seat tube and the down tube. The chain stay unit includes two first pivot points and two second pivot points. A first link unit has a first end pivotably connected to the first pivot points and a second end of the first link unit is pivotably connected to one of the two lugs. A second link unit has a first end pivotably connected to the second pivot points and a second end of the second link unit is pivotably connected to the other one of the two lugs. The distance between the bottom bracket and the rear wheel axle is not changed when the rear suspension system is activated.