The use of two feldspathic glasses with different thermal expansion coefficients (TEC) has been reported to improve mechanical properties of dental porcelains. A mismatch of TECs between the glasses is considered possible reason. Silica is main component of glass and has a four polymorphism. The TEC of fused silica is small and that of cristobalite is great due to transformation. The purpose of this study was to investigate the effect of adding polymorphism of SiO2 to the feldspathic porcelain on mechanical properties.