HRTEM image and structural simulation suggested that the observed nanostructure is r-P, which has been theoretically predicted to be stable but never experimentally observed. The r-P consists of alternate P8 and P2 structural units with variable ring diameters, observed inside the MWCNTs with inner diameters of 5–8 nm. A ring with a diameter of 5.30 nm, consisting of 23 P8 and 23 P2 units with a total of 230 P atoms, was observed inside a MWCNT with an inner diameter of 5.90 nm. The distance between neighboring rings is 6.4 Å. The surrounding graphitic walls of MWCNTs establish a nanoreactor for r-P structure formation and its protection against oxidation in air. A blue-shift of the G-band was observed from the sample containing both external and internal phosphorus, whereas a small red-shift was observed after removing most of the external phosphorus. The depth profile of P 2p XPS spectra of r-P@MWCNT shows that phosphorus oxides were distributed on the surfaces of the sample and could be etched away with an Ar ion beam. Observed P 2p core-level binding energies of r-P inside MWCNTs are smaller than external phosphorus.