than 500 nm results in failure to incorporate the ultrafine particles in the silicon nitride based crystal grains whereas that smaller than 1 nm results in failure to exert the composite effect after the incorporation thereof accordingly the size of the ultrafine particles is preferably in the range of 1 to 500 nm the foreign particles that are not incorporated in the crystal grains are dispersed and deposited in the grain boundary phase as particles of the order of nanometer in size according to the present invention as described hereinbefore the silicon nitride composite sintered body simultaneously improved in strength and fracture koughness is obtained by virtue of the composite effect in the crystal grains and or the grain boundary phase having the foreign particles of the order of nanometer in size 100 g of silicon nitride powder was compounded with each of various foreign particles in an amount to be dispersed as specified in table 1 and further with a sintering aid y 2 o 3 al 2 o 3 each in 5 by weight followed by wet mixing the obtained composite powder was subjected to cold isostatic pressing under a pressure of 1 5 t cm 2 into a molding followed by sintering at 1650 u00b0 c and an ordinary pressure in nitrogen for 10.