A series of o-cresol novolac epoxy (o-CNER)-based composites containing various amount of SBA-15, SBA-16, and MSU-X type mesoporous silicas were prepared, and their performances were evaluated. Morphological investigation by SEM reveals that the mesoporous silicas achieve a good dispersion in the o-CNER matrix due to an effective surface modification. The dielectric constants of all the composites were measured in the frequency range of 50 - 1,000 kHz. The investigation suggested that the dielectric constant could be reduced from 4.0 of the pure thermosetting o-CNER to 3.71, 3.73, and 3.73 by incorporating 5 wt.% SBA-15, SBA-16, and MSU-X, respectively. The reduction is attributed to incorporation of air voids stored within the mesoporous silicas, the air volume existing in the gaps on interfaces between the mesoporous silica and the matrix, and the free volume created by introducing large-sized domains. The composites present stable dielectric constants across the wide frequency range. An improvement of thermal stability of the o-CNER is achieved by incorporation of the mesoporous silica materials, while the enhanced interfacial interaction between the surface-modified mesoporous silica and the o-CNER matrix has also led to an improvement of toughness.