R-curve behavior of Si3N4-BN composites and monolithic Si3N4 for comparison was investigated. Si3N4-BN composites showed a slowly rising R-curve behavior in contrast with a steep R-curve of monolithic Si3N4. BN platelets in the composites seem to decrease the crack bridging effects of rod-shaped Si3N4 grains for small cracks, but enhanced the toughness for long cracks as they increased the crack bridging scale. Therefore, fracture toughness of the composites was relatively low for the small cracks, but it increased significantly to similar to8 MPa(.)m(1/2) when the crack grew longer than 700 mum, becoming even higher than that of the monolithic Si3N4.