During the process of water-based gelcasting of Si3N4, the gas discharged due to the reactions on the solid-liquid interface can lead to a great deal of big bubbles in the green bodies. To improve the process a type of method based on surface oxidation is developed. By means of the method a layer of SiO2 and Si2N2O, which can prohibit the gas from being discharged effectively, is formed on the surface of Si3N4 powders. Oxidized powders can be dispersed better than those not oxidized, and after gas-pressure-sintering mechanical property of the gelcast samples made from the oxidized powder is much better than that of dry-pressed and then isostatic pressed samples.