Development of a reliable sealant or sealing system remains one of the top priorities in planar solid oxide fuel cells (SOFC) technology. Glass matrix composites as sealing materials for SOFCs are known to enable design flexibility, e.g., by control of such properties as viscosity, seal operating temperatures, and visco-elastic properties. The objective of this study was to find suitable fillers for structural strengthening as well as for improving long-term cyclic sealability of the alkali borosilicate glass composite sealants. Alkali ion containing compliant sealing glass was developed by present authors. Two kinds of fillers, Al₂O₃ and 8 mol % Y₂O₃-stabilized zirconia (8YSZ), were used up to 15% for the preparation of glass composite sealants. The effects of these fillers and their content on the high temperature viscosities and electrical conductivities of the sealants were investigated. Furthermore, glass composite to STS444 seal assemblies were subjected to thermocycling between 450°C and 750°C. The composition of a glass composite sealant to survive over 12,000 h of consecutive cyclic test was identified. In this presentation, effect of fillers on properties and cyclic sealing behavior of the glass composites will be also discussed.