Ag-supported HZSM-5 and SiO2 were used as a catalyst for a combustion-type soot senor. Although Ag/HZSM-5 catalyst showed a higher intrinsic soot oxidation activity than that of Ag/SiO2, the soot sensor with Ag/HZSM-5 catalyst exhibited a much lower sensing performance than that with Ag/SiO2. This is because the Ag/HZSM-5 catalyst showed a quite low thermal conductivity, inhibiting the soot oxidation heat from transferring to the Pt detector inside of the sensor element. Therefore, improvement of the soot sensing performance depends on not only the excellent intrinsic soot oxidation activity of the catalyst but also the superior thermal conductivity of catalytic materials.