Mullite (3Al2O3.2SiO2) phase is the important stabile crystalline phase in the alumosilicate system. Mullite ceramics with high porosity serve as a heat insulator and filters, as well as can be used in constructional and thermal engineering. Zirconia ceramics also are excellent candidate for high temperature applications, because of high melting point (2715oC), good corrosion resistance and high mechanical properties. The glass phase of Al2O3-SiO2-ZrO2 system begins to form only from 1750 to 1850oC, which is higher than, for example, case of Li2O, Na2O and MgO. Therefore, the ZrO2 is used as modification additive and influences the properties of mullite ceramics