Abstract: High thermal conductivity silicon nitride ceramics were prepared by aqueous dry pressing process with water as dispersing medium and sintering at different temperatures (1 750, 1 850℃) using α-Si₃N₄, β-Si₃N₄, MgO, Y₂O₃ as raw materials. The structure, mechanical properties and thermal conductivity of the ceramics at different sintering temperatures were studied and compared with those of non-aqueous dry pressed silicon nitride ceramics with anhydrous ethanol as the dispersing medium. The results show that the grains of the ceramics were all long columnar, and there were many slender grains distributed around the scattered coarse grains, showing a dual-mode structure. At the sintering temperature of 1 750, 1 850℃, the bending strengths of aqueous dry pressed ceramics were 555.7, 747.5 MPa, and the fracture toughnesses were 8.14, 8.25 MPa·m^1/2, respectively, which were slightly lower than those of non-aqueous dry pressed ceramics; the relative densities were 99.00%, 99.58%, the average grain sizes were 1.06,1.27 μm, and thermal conductivities were 65.70,75.54 W·m^-1·K^-1, respectively, which were slightly higher than those of non-aqueous dry pressed ceramics.