The preparation of pore gradient silicon nitride (Si₃N₄) ceramics was investigated by thermally spraying dense Si₃N₄ coatings onto the porous Si₃N₄ ceramic substrates via a high-velocity oxyfuel (HVOF) spraying method. Spray powders with excellent processability were developed and produced by spray drying and sintering in a nitrogen atmosphere. After sieving, the powders with average particle size of 45-75 μm were selected. Optimization of spray parameters such as spray distance and hydrogen flow rate was carried out in order to find the most decisive factor necessary for the production of dense and well-adhering coatings and pore gradient stuctures. Zirconium phosphate bonded Si₃N₄ porous ceramics were used as the substrate and coatings were obtained only when spray distance was 200 mm. A pore gradient structure was obtained at lower H₂ flow rate, while poor-adhering coatings were obtained at higher H₂ flow rate due to the etching. The hardness is significantly improved after Si₃N₄ coatings were successfully applied.