We have fabricated porous silicon layers (PSLs) in the anodizing solution, HF(48wt.%): (\mathrm{H}_{2} \mathrm{O}=1: 1). After anodization, the PSL was etched in the same chemical solution as used for anodization and then illuminated with the (476.5 \mathrm{~nm}) line, with a power of (20 \mathrm{~mW}), from an argon ion laser. A few minutes later we observed its photoluminescence (PL) spectrum under the same excitation power and then repeated the process of chemical etching followed by laser illumination, many times on the same specimen. For comparison, some spots on one PSL were etched for different times then to obtain a PL spectrum each spot was illuminated once by laser. The PL spectra of PSLs treated by the above two processes are very different. The PL band of the PSL treated by etching and illumination alternatively exhibited a continuous increase in intensity along with a blue shift for the first 5 repetitions. After reaching a maximum, the PL band exhibited a continuous reduction in intensity along with a red shift. Comparing the PL spectra for the different spots of the PSL treated for different etching times, the energy positions of their PL bands were almost the same. Only the intensity increased slightly with the time of etching. We think the blue and red shifts of the PL band for the PSL treated by etching combined with illumination are related to the quantum size effect of the PSL.