The fracture energy of a porous silicon nitride with aligned fibrous grains was investigated, using a chevron-notched-beam technique. A crack was constrained to propagate normal to the grain alignment. The obtained fracture energy was similar to 500 J/m(2), which was similar to 7 times larger than that of a dense silicon nitride with randomly oriented fibrous grains. The large fracture energy was attributable primarily to the sliding resistance associated with interlocking grains.