There are provided a process for forming a silicon nitride sintered body, encompassing a sialon sintered body, by making much of the superplasticity of the sintered body intact as a simple material without formation thereof into a composite material, and a formed sintered body produced by the foregoing process. A silicon nitride sintered body (encompassing a sialon sintered body) having a relative density of at least 95% and a linear density of 120 to 250 in terms of the number of grains per 50 .mu.m in length in a two-dimensional cross section of the sintered body is formed through plastic deformation thereof at a strain rate of at most 10.sup.-1 /sec under a tensile or compressive pressure at a temperature of 1,300 to 1,700.degree. C. The formed sintered body has a degree of orientation of 5 to 80% as examined according to a method specified by Saltykov, a linear density of 80 to 200, and excellent mechanical properties especially at ordinary temperatures.