A dense, self-reinforced silicon nitride ceramic prepared by pressureless sintering or low pressure gas sintering. The composition comprises (a) silicon nitride, at least 20 percent of which is in the form of .beta.-silicon nitride whiskers having a high average aspect ratio, (b) from about 2 to about 10 weight percent of a glassy grain boundary phase containing magnesium oxide, yttrium oxide, silica, zirconia, and optionally, one oxide selected from the group consisting of calcium, gallium, indium and hafnium oxides, and (c) a second crystalline phase in an amount ranging from 0.5 to about 5.0 weight percent comprising zirconium oxide, and (d) optionally, crystalline phases of metal zirconium silicide and/or metal zirconium silicon nitride in a combined amount from 0.1 to 3.0 weight percent, wherein the metal is tantalum, calcium, hafnium, gallium or indium, and wherein when the metal is calcium, hafnium, gallium or indium, the same metal occurs as the metal oxide in the glassy phase. The ceramic exhibits high fracture toughness and high fracture strength and has a density of at least 98 percent of theoretical.