Porous thin films containing very small closed pores (similar to20 Angstrom) with a low dielectric constant (similar to2.0) and excellent mechanical properties have been prepared using the mixture of cyclic silsesquioxane (CSSQ) and a new porogen, heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (tCD). The pore sizes vary from 16.3 Angstrom to 222 Angstrom when the content of tCD in the coating mixture increases to 45 wt.-% according to positronium annihilation lifetime spectroscopy (PALS) analysis. It has also been found that the pore percolation threshold (the onset of pore interconnectivity) occurs as the similar to50% tCD porogen load, The dielectric constants (k=2.4similar to1.9) and refractive indices of these porous thin films decreased systematically as the amount of.porogen loading increased in the coating mixture. The electrical properties and mechanical properties of such porous thin films were fairly good as interlayer dielectrics.