Ultimate one-dimensional heterojunctions of electron donor and acceptor materials have been realized within the inner hollow space of a single-walled carbon nanotube (SWNT). The heterojunction structures of Cs/I and Cs/ C60 inside SWNTs (Cs/I@SWNTs, Cs/ C60 @SWNTs) yield the air-stable rectifying performance. Clear tunneling currents through the p-n junction barrier could be also detected only for Cs/I@SWNTs, which is explained by the difference of depletion layer structures. Based on a potential calculation, symmetrical and asymmetrical depletion layers were found to be formed in Cs/I@SWNTs and Cs/ C60 @SWNTs, respectively. Low temperature measurements also supply evidence of asymmetric depletion layer formation in Cs/ C60 @SWNTs.