A process for preparing a self-reinforced silicon nitride ceramic body of high fracture toughness comprising hot-pressing a powder mixture containing silicon nitride, a densification aid such as sodium oxide, a conversion aid such as lanthanum oxide and a compound, such as gallium oxide, which enhances growth of .beta.-silicon nitride whiskers-under conditions such that densification and the in situ formation of .beta.-silicon nitride whiskers having a high aspect ratio occur. A novel silicon nitride ceramic of high fracture toughness and high fracture strength is disclosed comprising a .beta.-silicon nitride crystalline phase wherein at least about 20 volume percent of the phase is in the form of whiskers having an average aspect ratio of at least about 2.5; a glassy second phase containing the densificaton aid, the conversion aid, the compound which enhances growth of .beta.-silicon nitride whiskers, and an amount of silica; and not greater than about 10 weight percent of the total weight as other phases. The glassy phase may also include a minor amount, e.g., up to 5.0 weight percent, based upon total weight of the ceramic, of aluminum nitride or boron nitride. The glassy phase optionally includes an amount of a secondary reinforcing material such as silicon carbide whiskers.