The aim of this study is to investigate fracture toughness of alumina-mullite composites produced by infiltration process. Alumina-mullite composite samples were fabricated using a hydrolyzed ethyl silicate sol (TEOS) which was infiltrated as SiO2 source into alumina preforms. Infiltrated samples were cold pressed isostatically at 150 MPa at vacuum followed by presintering at 1200degreesC for one hour. Final sintering was carried out at 1650 degreesC for 4 hours. Phases formed after sintering were determined by XRD revealing alumina and mullite phases. Fracture toughness measurements were performed by using indentation technique indicating that fracture toughness of samples was increased with raising mullite content.