Effects of crystallization on the high-temperature mechanical properties of a newly developed silicate-based glass sealant (GC-9) are investigated for use in planar solid oxide fuel cell (pSOFC). The aged, crystallized GC-9 glass is produced by heat treatment of the original GC-9 glass at 900 degrees C for 3 h. Not only crystalline phases are formed but the residual glass is also changed in the aged GC-9 glass after the heat treatment. Mechanical properties of the aged GC-9 glass are determined by four-point bending technique at temperature from 25 degrees C to 750 degrees C. The glass transition temperature of the given glass is reduced but the softening temperature is increased by such a crystallization heat treatment. The aged GC-9 glass exhibits a greater flexural strength and Young's modulus than the non-aged one at temperature below 650 degrees C due to the existence of crystalline phases. At temperature of 700 degrees C and 750 degrees C, a greater extent of stress relaxation is found in the aged GC-9 glass such that its strength and stiffness are much lower than those of the non-aged one. The changes in the thermal and mechanical properties through the given aging treatment are favorable for application of the GC-9 glass sealant in pSOFC. (C) 2009 Elsevier B.V. All rights reserved.