A technique for fabrication of beta ' -SiAlON-based ceramics in three-dimensional woven fabrics of BN-coated SiC (Nicalon (TM)) fibers was developed by reactive melt infiltration in a controlled N-2 atmosphere. beta ' -SiAlON was produced in situ by the reaction of beta -Si3N4, AlN, and Y-Al-Si-O molten glass. The wettability of the fibers with the molten glass was improved by infiltration and pyrolysis of perhydropolysilazane, resulting in fully dense matrix composites. The reaction between the fiber and molten glass could be depressed by increasing the N2 partial pressure during the melt infiltration. The inhibition of the interfacial reaction may be related to the formation of carbon and oxynitride on the SiC fiber, in agreement with thermodynamic calculations as a function of N2 partial pressure. The fabricated composites had a high ultimate flexure strength and a large work of fracture at room temperature. Degradation of the mechanical performance of the composites was small, even at 1773 K in an argon atmosphere.