Using Si powder with average particle size of 8${mu}{ extrm}{m}$ Si compacts were formed by pressureless powder packing method. The compacts were reaction bonded at 1350, 140$0^{circ}C$ for 3~35 hrs under N2/H2 atmosphere and its microstructures were examined. Reaction bonded silicon nitrides showed nitridation of 90% and relative density of 88% After the impregnation of 5wt% MgO as sintering additive using aqueous solution of Mg nitrate the Si compacts were reaction bonded at 140$0^{circ}C$ for 15hrs. The reaction bonded bodies were gas pressure sintered at 180$0^{circ}C$ 190$0^{circ}C$ 200$0^{circ}C$ for 150, 300min. They showed relative density of 95% bending strength of 600MPa and fracture toughness of 6 MPa.m1/2