A portable bicycle comprises a front bar having a front end coupled to a head tube, a rear frame section including a lower seat tube extended upwardly from a bottom bracket, a upper seat tube spaced and aligned with the lower seat tube, and a hollow rear bar for slidably receiving the front bar, the rear bar being intersected the upper and lower seat tubes, and a fastening mechanism for securing the front bar to the rear bar. Detaching a seat post and a saddle, unfastening the fastening mechanism, fully projecting the front bar from a rear opening of the rear bar, and fastening the fastening mechanism will shorten the distance between front and rear wheels to a minimum and facilitate the bicycle to store or carry in a much reduced size.