In this paper, Al2O3-SiO2-ZrO2 (ASZ) composite was developed via pressureless sintering route and investigated for microstructures and mechanical properties. The fexural strength and fracture toughness of the SiO2 + ZrO2 particles reinforced alumina matrix was developed and measured 230.39 MPa and 2.39 MPa.m1/2, respectively, demonstrating a signifcant tougheners due to the presence of both particles. The microstructural observation on cracked and fractured samples showed that three toughening mechanisms, crack defection, crack bridging and microcracks were the contributing factors for the enhanced toughness of the matrix and the composite. Besides, phase transformation toughening and residual stress due to difference of coeffcient of thermal expansion played a key role for the improved toughness of the composites.