In this paper, Si3N4 fiber materials were fabricated by nitridation of porous Si green bodies, which were prepared by the foaming method combined with gel-casting. The effect of the nitriding process (the heating rate and the nitrogen flow rate) on the phase and the microstructure was studied. The results show that decreasing the heating rate and the nitrogen flow rate is both beneficial to the growth of Si3N4 nanofibers and promoting the disintegration of the pore wall structure of the porous green bodies. The optimized nitrogen flow rate and the heating rate for the growth of silicon nitride fibers are 150 mL/min and 0.5 °C/min, respectively.