Relationship between microstructure and fracture behavior of monolithic Si₃N₄ and Si₃N₄/25vol%SiC nanocomposites was investigated. For the Si₃N₄/25vo1%SiC nanocomposites, the fracture strength was increased by the nano-size SiC dispersion which allowed to control the growth of fine and uniformly elongated Si₃N₄ grains. Fracture toughness was also improved because of bridging and pull-out of elongated Si₃N₄ grains. The variation of the fracture toughness value with the indentation load, i.e. crack length, was measured by indentation fracture (IF) method, and this result was compared with that of chevron notched beam (CNB) method. In this experiment, the fracture toughness by IF method exhibited a rising with increasing the crack length. This was interpreted as due to the rising R-curve behavior also observed by the CNB method. Therefore, it is proposed that R-curve behavior can be also estimated by IF method. However, the saturated K_R value observed by IF method was within ±15% of measured by CNB method.