A cycle frame easily converted from a bicycle to a tricycle and vice versa, and to a cycle and a baby carriage using same. The cycle includes a main frame including a main support and a pair of sub-supports branching from the main support, a pair of conversion frames, front end portions thereof being hingeably connected to rear end portions of the sub-supports such that the distance between rear end portions thereof is adjustable, a pair of connection brackets respectively connected to the rear end portions of the conversion frames, a fork frame rotatably connected to the front end portion of the main support, a front wheel shaft rotatably connected to the fork frame, a main rear wheel shaft, both ends thereof being respectively and separably connected to the connection brackets, and a propelling pedal rotatably mounted on the main support and drive-connected to the main rear wheel shaft.