We demonstrate the possibility to characterize Si photonic crystal (PhC) nanocavity modes made on Silicon on insulator (SOI), and operating at telecom wavelengths, through photoluminescence (PL) spectroscopy at room temperature. In fact, a wide PL band between 1200 and 1600 nm is observed under optical pumping when proper material processing is performed after the nanocavities fabrication, namely Ar/H 2 plasma treatment and Si implantation. PL emission is originated through the carrier recombination occurring at defect states formed in silicon after such treatments. We study photonic modes of L3 PhC nanocavities, with a proper geometry optimization in order to obtain high quality (Q) factors [1] and improved coupling to the far field [2].