A tension adjusting device for a transmission system of a bicycle includes a rear frame pivotally and linearly mounted to a front frame of the bicycle, the rear frame including a pair of arms parallel to each other and connected to each other. Each arm includes a rear end for mounting a the rear wheel of the bicycle and a front end extending to the front frame of the bicycle and connected by an axle that is pivotally mounted in a the front frame. An adjuster is mounted between the two arms. The adjuster includes a linkage includes a lower end pivotally connected to the two arms and a bolt screwed through an upper end of the linkage and extends into the front frame to make the upper end of the linkage abut against the front frame due to a gravity from the bicycle.