Silicon nitride containing 2 w/o Y2O3 and 1 w/o Al2O3 as sintering additives was fabricated by gas pressure sintering and the effect of grain growth onthe flexural strength and fracture toughness was investigated. Duplex microstructure consisting of the large grains and the matrix of small grains was obtained by the two step gas pressure sintering : 180$0^{circ}C$ for 2 h in a N2 atmosphere of 2 MPa followed by 190$0^{circ}C$ for 0 to 8 h in a N2 atmosphere of 8 MPa. The average diameter and aspect ratio of the matrix grains were not changed with the second step sintering time, The flexural strength and fracture toughness were dominated by the large grains. The flexural strength was decreased with sintering time due to increasing the size of large grains which was acted as a fracture origin. The fracture toughness was increased with sintering time due to increasing the diameter and aspect ratio of the large grains and then decreased when the sintering was performed more than 4 h due to the transgranu-lar fracture of the large grains which have a diameter of >~2 ${mu}{ extrm}{m}$