We have sysmatically studied the porous silicon (PS) that emits blue light with a peak energy at 2.7 eV; measuring its time-resoluted photoluminescence (PL) spectra and the local vibration mode absorption, and studying the Gamma ray lrradlatlon effect on its PL spectra and comparing the effect with that for the PS which emits red or yellow light. We have also studied the stabability of the blue light emission from PS after long term storage in air and irradiation with laser and ultroviolet light. our experimental resuits and those in other references for blue light emission from PS are contradictory with the quantum confinement model, but can be explained by the quantum confintjnlent/luminescence centers model. We consider that the blue light emission originates from some characteristic luminescence centers in the SiOx layer covering nanoscale silicon in PS.