designed with functions in the three dimensioned structure the articulation mechanism has rigidity in horizontal and is free in vertical that is the articulation mechanism can not only keep the primary bicycle body and the auxiliary bicycle body in parallel but also allow the primary bicycle body and the auxiliary bicycle body moving stably in different levels such as driving over an uneven or bumpy road with some drops in height the wheels of the primary bicycle body and the auxiliary bicycle body can move vertically and in parallel to stabilize and balance the twin frame bicycle thus solving the problem of tumbling or overturning when driving over an uneven or bumpy road and or making a turn besides providing fun riding the articulation mechanism linking the primary bicycle body and the auxiliary bicycle body makes the twin frame bicycle stable and unlikely to tumble or overturn moreover by utilizing the articulation mechanism together with a resilient tension mechanism having an elastic element the twin frame bicycle can always stand straight up and be controlled much smoothly furthermore since the structure of the auxiliary bicycle body is simple the cost is saved and the maintenance of the twin frame bicycle is easy in addition since the auxiliary wheel can be disposed between the front wheel and the rear wheel of the primary bicycle body the center of gravity of the twin frame bicycle will not be located at.