Abstract: A silicon nitride based sintered product having strength at a high temperature is obtained by sintering a mixture. The mixture comprises a silicon powder or a mixture powder of silicon and silicon nitride added a sintering assistant powder, a compound powder of iron and a compound powder containing at least one of vanadium, niobium and tantalum in 5a group elements of the periodic table. The silicon nitride based sintered product contains silicon, aluminum, yttrium, oxygen, nitrogen, iron and at least one of 5a group element in the periodic table, and when weights at which the elements are present as a compound of silicon nitride, alumina, yttria, a compound powder of iron and 5a group elements in the periodic table are represented by a, b, c, d and e, the following formulae are fulfilled:1.ltoreq.100(b+c+d+e).times.(a+b+c+d+e).ltoreq.15, 1.ltoreq.(b+c)/d0.005.ltoreq.100d/(a+b+c+d+e).ltoreq.7.5, and 0.005.ltoreq.e/(b+c).ltoreq.