Silicon nitride specimens consisting of silicon nitride powder, SN-E10, and 8-10 mass % of yttria and alumina additives were placed in a set of specially designed alumina crucible filled with various kinds of silicon nitride and alumina packing powders. The silicon nitride ceramic was successfully sintered without serious mass loss in air furnace at temperatures ranging from 1550 to 1700 ºC for 1 to 2 hours. The final relative density of 98 % was achieved when sintering silicon nitride specimen containing 10 mass % additive at 1700 ºC for 2 hours. The average values of Vickers hardness (Hv), fracture toughness (K1c) and flexural strength (ASTM F-294) were 15.9 GPa, 5.0 MPa.m1/2 and 420 MP2, respectively. Moreover, low cost powders with weak agglomeration after sintering suitable for use as packing powders, were SN-F2 for silicon nitride and A-11 for alumina. Despite the compatible hardness and toughness, the value of flexural strength is was relatively low compared to 500-600 MPa of commercial Si3N4 ceramics.