Silicon nitride (Si3N4) ceramics have been interested for electrical substrate applications, because the ceramics can be made highly mechanical strength, fracture toughness, electrical resistivity and high thermal conductivity. Generally, relatively large amount of additives are required to obtain dense Si3N4 ceramics. During sintering, additives react with SiO2 including surface oxide of Si3N4 raw powder to form a liquid phase. Most of liquid phase changed into glassy phase during cooling down. In this study, Si3N4 ceramics were fabricated by gas pressure sintering. Yttrium oxide (Y2O3), silica (SiO2), and magnesia (MgO) were used for liquid-phase-enhanced sintering process. Dense materials were sintered by this process, but their thermal conductivities were not so high (30-40 W/m·K). Therefore, post-sintering heat-treatment process was performed to reduce the excess amount of glassy phase. An additive system (3 mass% SiO2 with 3 mass% MgO and 1-5 mass% Y2O3) was selected as the sintering aid. These ceramics could be sintered to almost full density at relatively low temperature as 1650oC for 2 h under 0.1 MPa-N2 without packing powder. The resulting materials have high bending strength, about 1 GPa, when 5mass% of Y2O3 was added. Based on the creation of low temperature pressureless sintering without packing powder, a novel two-step sintering (once firing) was proposed. The two-step sintering conducted by sintered at 1650oC under 0.1 MPa-N2 for 2 h for densification in the first step. Followed by heated up to and kept at 1950oC for 8 h under 1.0 MPa-N2 in the second step. The Si3N4 ceramics could be fabricated with relatively high thermal conductivity of 90 W/m·K. Mass loss, microstructure, mechanical properties, oxygen content and chemical composition were discussed.