A foldable bicycle frame includes a substantially vertical center tube at a mid portion of a bicycle frame, an upper center tube and a lower center tube to be partially fitted in upper and lower ends of the center tube, respectively. A front half of the bicycle frame includes the center tube and a rear half of the bicycle frame include the upper and lower center tubes, thereby enabling the front and rear halves to be folded or extended about the vertical center tube into a folded position or running position. The bicycle frame comprises radially annularly arranged continuous V-shaped teeth on connection surfaces of the upper and lower center tubes and the center tubes, and clamping means for clamping the center tube and the upper and lower center tubes in engagement with the teeth on the connection surfaces, thereby enabling the bicycle frame to be folded or extended by releasing and then clamping the center tubes. The bicycle frame according to the invention further comprises positioning means for positioning the front and rear halves in the folded and extended positions, lever means for fixing the halves by one operation, and locking means for the lever to avoid a risk of releasing the clamp of the halves when running.