In this work, we carry out a comprehensive photoluminescence (PL) study to elucidate the origin of light emission from porous silicon nanowires (pSiNWs). SiNWs were first grooved in lightly doped Si wafer by silver assisted chemical etching, and then treated with an acid vapor emanating from HF/HNO3 aqueous solution heated at 60 °C. Scanning and transmission electron microscopies were used to investigate the effect of the acid vapor etching on morphological properties of SiNWs. The as prepared pSiNWs exhibited a strong room temperature PL emission centered at 1.93 eV. An increase of the PL intensity was observed with the increase of HNO3 in the acid solution. By varying the laser excitation density from 60 to 300 W/cm2, we shed the light on the radiative recombination modes occurring within the Si nanocrystals (SiNCs) generated along the pSiNWs. We study as well the temperature-dependent PL of the pSiNWs in the range 10 to 300 K. Based on both laser excitation density and temperature-dependent PL, we propose a multilevel transition scheme resuming the PL origin taking into account the size distribution, shape and surface states of the SiNCs trimming the wire sidewalls.