High thermal conductivity silicon nitride ceramics were prepared via aqueous gel-casting with a slurry of a low viscosity. The effect of MgO sintering aid on the rheological properties of slurry and the thermal conductivity of silicon ceramics was investigated. The results show that MgO sintering aid causes the particles aggregation and increases the slurry viscosity. The slurry viscosity becomes sensitive to the pH value change when the addition amount of MgO increases. A ceramic slurry with a low viscosity is achieved by adjusting pH value and dispersant concentration. The thermal conductivity, bending strength, and fracture toughness of the sample prepared by aqueous gel-casting can reach 83.7 W·m-1·K-1, 945 MPa, and 8.4 MPa·m1/2, respectively. The feasibility of high thermal conductivity silicon nitride ceramics prepared by aqueous gel-casting was verified.