Mullite-boron nitride (BN) composite with high strength, low Young's modulus, and highly improved strain tolerance was prepared by reactive hot pressing (RHP) using aluminum borates (9Al(2)O(3).2B(2)O(3) and 2Al(2)O(3).B2O3) and silicon nitride as starting materials. Compared with the monolithic mullite, the composite RHPed at 1800degreesC showed 1.64 times (540 MPa) the strength, 70% (153 GPa) the Young's modulus, and 2.34 times (3.53 X 10(-3)) the strain tolerance. Transmission electron microscopy observation revealed that the composite had an isotropic microstructure with a fine mullite matrix grain size of less than 1 mum and nanosized hexagonal BN (h-BN) platelets of about 200 nm in length and 60-80 nm in thickness. The high strength was suggested to be from the reduced matrix grain size and the small toughening effect by the h-BN platelets.