This paper demonstrates 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 plasma treatment and Si implantation. PL emission is originated through the carrier recombination occurring at defect states formed in silicon after such treatments. Photonic modes of L3 PhC nanocavities were studied with a proper geometry optimization in order to obtain high quality (Q) factors and improved coupling to the far field.