Starting with a mixture of ultrafine beta-Si3N4 and a SiO2-containing additive, a superplastic Si3N4-based composite was developed, using the concept of a transient liquid phase. Significant deformation-induced phase and microstructure evolutions occurred in the nonequilibrium, fine-grained Si3N4 material, which led to the in situ development of a Si3N4 22-vol%-Si2N2O composite and strong texture formation. The unusual ductility of the composites with elongated Si2N2O grains was attributed to the fine-grained microstructure, the presence of a transient liquid phase, and the alignment of the elongated Si2N2O grains, The mechanical properties of the resultant composite were enhanced rather than impaired by superplastic deformation and subsequent heat treatment; the resultant composite exhibited both high strength (957 MPa) and high fracture toughness (4.8 MPa.m(1/2)).