Silicon nitride has been widely employed as one of the most important engineering ceramics for many practical applications due to their excellent properties such as high temperature resistance, low density, high corrosion resistance and other mechanical properties. In this study, silicon nitride (Si3N4) ceramic materials was synthesized by a direct nitridation method of amorphous silicon powder under a flow mixture of argon and nitrogen or ammonia. The amorphous silicon powder was placed in a tube furnace at the temperature of from 1300oC to 1600oC for 30-800 min under a flow of gases mixture. The phase compositions and morphology of the obtained Si3N4 powder were characterized by using X-ray diffractometry (XRD) and SEM techniques, respectively. The a-Si3N4 formed at temperature of 1500oC under a mixed gases flow of 3 L/ min for 400 composed of straight rod-like fibers with a length in the range of 5 to 100 mm and diameters of about 0.3-4 mm. The effects of gas flow rate, conversion temperature, reduction time and gas compostion on the effectiveness of a-Si3N4 formation were investigated in detail. Effect of the auxiliary gases and synthesizing conditions for direct nitridation of Si3N4 on phase content and phase transformation of a-Si3N4 to β-Si3N4 were also evaluated.