R-curves for two in situ reinforced silicon nitrides A and B, of different microstructures, were characterized using indentation-crack growth measurements. Silicon nitride B, with its coarser microstructure and 8 MPa m(1/2) tough ness, showed higher resistance to crack growth and more damage tolerance than silicon nitride A, with its finer microstructure and 7 MPa m(1/2) toughness. However, silicon nitride A showed a higher Weibull modulus than that of silicon nitride B due to the relatively narrow critical grain-size distribution. These results suggest that a coarse microstructure with narrow flaw-size distribution is beneficial to toughening, damage tolerance, and reliability.