Silicon nitride sintered bodies including Si.sub.3 N.sub.4 and at least one element selected from Y.sub.2 O.sub.3, MgO, ZrO.sub.2. In the silicon nitride bodies, a J phase solid solution is present in an intergranular phase, which satisfies a relation of 0.65x.ltoreq.y.ltoreq.x+2 wherein y is an amount of J phase solid solution and x is an amount of Y.sub.2 O.sub.3 as an additive.