Nitrogen-doped mullite fibers were first synthesized through the nitridation of Al2O3–SiO2 gel fibers in NH3. The results showed that nitrogen take-up began at 800°C, reached the maximum at 900°C, and then decreased with increasing temperature. The ceramic fibers nitridated at 900°C were essentially amorphous, but contained a small amount of nano-sized Al–Si spinel crystals. Mullite was formed after nitridation at 1200°C, accompanied by crystallization of χ-SiAlON and δ-Al2O3. The incorporation of nitrogen resulted in the formation of a variety of nitrogen-containing crystalline phases. The grain size of the mullite fibers can be adjusted by changing of the nitrogen content.