High-volume-fraction Si3N4-Al-based composites have been fabricated by high-pressure casting method. The effects of chemical compositionof infiltrated Al alloys, microstructures of Si3N4 preforms, and test temperature on the mechanical properties were investigated.The maximum four-point bending strength and fracture toughness of the composites reached 924 and 8.2MPa(√m), respectively. Increasingsintering temperature above 1650℃ for Si3N4 preforms resulted in an enhancement of fracture toughness at the expense of degradation inflexural strength. However, an increase in sintering time for Si3N4 preforms at moderately sintering temperature yielded an improvement inboth flexural strength and fracture toughness of the corresponding composites. Both flexural strength and fracture toughness decreased withincreasing temperature and CIP pressure due to inhomogeneous distribution of Al phase and some defects introduced into the preforms duringthe casting process. The Si3N4–6061 Al composite exhibited the lowest strength, which may be attributed to the presence of porosities andinterfacial reactions.