Y-TZP/mullite composites were prepared by the infiltration of Y-TZP precursor into partially reaction-sintered mullite. The addition of Y-TZP(~7.2 wt%) increased the bend strength(207 MPa), fracture toughness(4.6MPa.m1/2) and Vickers microhardness(853kg/$ extrm{mm}^2$) of the uninfiltrated mullite sintered at 162$0^{circ}C$ for 10h by more than 75, 70 and 105%, respectively. Residual alumina-rich glass was observed at a mullite/mullite junction, due to the mullitization reaction of silica melt with crystalline $alpha$-Al2O3 during a final sintering. Although ZrO2 inclusions improved the final sintered density of mullite they did not effectively prevent its grain growth.