The effects of mesoporous silica (MS) addition with a low thermal conductivity on the thermoelectric properties of Nb-doped SrTiO3 bulk materials are discussed. Nb-doped SrTiO3 bulk materials with varying amounts of added MS were fabricated by the conventional normal pressure sintering method in an Ar atmosphere. MS addition reduced the thermal conductivity, but enhanced the electrical conductivity significantly. Meanwhile, the Seebeck coefficient was almost independent of MS addition, thus, enhancing the ZT value. The sample with 5vol% MS gave the maximum ZT value of 0.165 at 900K. The reason for the reduced thermal conductivity and enhanced electrical conductivity by MS addition was also investigated.