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BN/Si 3 N 4 composite ceramics with different BN content were prepared by water-based gel casting process with raw and Y–Al-oxides-coated BN powder, separately. A Y–Al oxides layer with the thickness ranging from dozens to hundreds of nanometers, serving as sintering additives, was coated on the surface of BN particles with the method of precipitation of inorganic salts. As a result, the fluidity of BN/Si 3 N 4 slurry with Y–Al-oxides-coated BN powder was much improved compared to that with raw BN. SEM examination showed the Y–Al-oxides-coated BN particles were homogeneously distributed within the Si 3 N 4 grain boundaries, and BN aggregation was barely detected. The BN/Si 3 N 4 composite ceramic prepared by Y–Al-oxides-coated BN powder showed a better sintering behavior than those prepared by raw BN due to the formation of fine and homogeneous microstructure in it. Compared to the BN/Si 3 N 4 ceramic with 15 vol% raw BN, the density and flexural strength of the ceramic with the same content of surface-coated BN were significantly improved to 8.5% and 67.2%, respectively. [ABSTRACT FROM AUTHOR]