We studied the effect of pore diameter on the film properties of low-density porous material with special interest in the pore diameter range below 5nm. A novel low-density material, Interpenetrated SOG (IPS), was designed to realize such porous character. It is composed of a pyrolic template, which is an organic olygomer, and framework, made from SOG polymer and silica sol, forming an interpenetrated structure. Thermal decomposition of the organic component successfully resulted in films with pore diameter range below 5nm. It is found that the dielectric constant and the pore diameter depended on the density and the film strength was affected by the average pore diameter. Practically, films with the average pore size below 5nm possessed dielectric constants as low as 2.2–2.0 and sufficient strength.