Whiskers of alpha- and beta -Si3N4 were grown on or near a carbon black substrate, respectively, 10 mm downstream from a mixed starting powder of low-grade silica and carbon, in flowing nitrogen gas at 1400 degreesC. The parameters (flowing nitrogen gas rate, growth time, grade of silica, and type of carbon) that promoted growth of the whiskers were examined in view of increasing the whisker yield. The shapes and sizes of both types of whiskers were observed by scanning electron microscopy (SEM). The separate growth of the whiskers is discussed here, based on X-ray diffraction analysis and SEM observation with energy-dispersive X-ray analysis.