A method of unidirectionally aligning the whiskers during tape casting and a sintered silicon nitride laminate of controlled properties and microstructure through alignment of the reinforcing grains fabricated by using the method are disclosed. The whiskers are unidirectionally oriented by placing a row of guides 0.2-0.5 mm apart from each other at exit of the reservoir of the slip containing the whiskers during tape casting. The silicon nitride comprising 10-30 volume % of elongated large grains growing from the .beta.-Si.sub.3 N.sub.4 whiskers and oriented unidirectionally in the fine grained matrix is fully densified by gas pressure sintering at 1900.degree. C. It exhibits a fracture toughness value as high as 9.24 Mpa m normal to the whisker alignment according to the Evans-Charles' method disclosed in the Journal of the American Ceramic Society, 59 [7-8], 371-372 (1976).