To advance the design of composite materials, it is important to understand their mechanical behaviors. This study presents the results of experimental measurements of the mechanical behaviors of particulate epoxy-alumina composites. Fracture toughness is measured using three-point bending. Fracture paths are studied using micro- X-ray computed tomography. The effects of ceramic filler volume fraction and size on the fracture toughness and fracture mechanisms are studied. Furthermore, the implications are discussed for designing robust composite materials.