A nanotube diode fabricated from a single C/CNx multiwalled nanotube exhibits a large photocurrent and a large photovoltage under illumination. The current-voltage (I-V) characteristics of the diode indicate a clear rectification effect. By comparing the I-V characteristics of C, CN x, and C/CMx nanotube diodes, we show that the rectifying characteristics of the C/CMx diode arises from the molecular junction formed at the C/CNx interface where the C and CNx segments are chemically bonded. External radiation photochemically generates electrons and holes in the C/CNx nanotube, producing a large photocurrent because of the influence of the strong electric field in the vicinity of the C/CNx junction. These unique photoresponsive characteristics of C/CNx nanotube junction diodes points to potential applications such as photovoltaic devices and photodiodes.