We have developed glass/ceramic composite seals composed of glass precursors (NS particles) and ceramic precursors (S particles). The composite seals exhibited low gas permeability at room temperature −650°C, irrespective of the fusion profiles. However, short-term fusion led to the formation of pores in the seals and the pores in the seals depressed the thermal cycle durability. As for the NS/S composite ratios, the addition of S particles obviously improved the thermal cycle durability, but excess addition will bring about delamination at the seal/substrate interface. Appropriate NS/S ratios as well as an adequate fusion profile would result in composite seals with high strength and adhesion.