The microstructure of silicon nitride containing different percentages of beta-seeds was investigated. The average grain size and volume fraction of large grains increased with the incorporation of beta-seeds. The length and aspect ratio of large grains in sintered Si3N4 initially increased by incorporating beta-seeds, and then decreased. Similar trends were observed in the fracture toughness. The toughening mechanisms and fracture behaviors were correlated with the grain morphology measured from image analysis. Plots of measured toughness versus volume fraction of large grains fit very well with the equation derived by Becher and Budiansky, which strongly suggested the essential role of debonding length in the toughness of silicon nitride. Further improvement of fracture toughness in silicon nitride may be possible by tailoring the amount of incorporated beta-phase seeds and leading to optimize the volume fraction, grain size, and aspect ratio of the elongated grains.