This work studies the effects of microstructure and relative density on the dielectric constant of porous silica prepared by two different techniques: gelcasting of suspension (GCS) and gelcasting of foams (GCF). Both routes are carried out from the preparation of silica suspension containing starch granules. In GCS route, the resultant suspensions were heated for consolidation, while in GCF route, the suspension was foamed before consolidation. The results indicated that the dielectric constant of porous silica was obviously determined by relative density, but scarcely influenced by microstructure. The logarithmic fitting equation lnε = ρ·lnεs was a good model to describe the relationship between the dielectric constant and the relative density of porous silica prepared by both GCS and GCF routes.