Different microstructures of the 70% Si3N4-30%BAS self-reinforced composite, fine, coarse and bimodal, are obtained by pressureless sintering at 1920degreesC. Flexural strength, fracture toughness and crack-growth resistance (R-Curve) behavior of each microstructure are characterized by three-point bending, indentation and modified compact tension methods respectively, at room temperature. The crack deflection, whisker bridging and pullout are considered as major toughening mechanisms in the composite. It is found that coarsening beta-Si3N4 whiskers of this composite can improve the toughness/fracture resistance but deteriorate the strength. When limited large abnormally grown whiskers are introduced into the microstructure, the composite shows an improved toughness/fracture resistance behavior and concurrently sustains a high strength.