We demonstrate that individual single-walled carbon nanotubes (SWNTs) can form ideal p-n junction diodes. An ideal behavior is the theoretical limit of performance for any diode, a highly sought after goal in all electronic materials development. We further elaborate on their properties by examining photovoltaic effects, an application where its performance is intimately related to the quality of the diode. Under illumination, SWNT diodes show significant power conversion efficiencies owing to enhanced properties of an ideal diode.