Nanocrystalline alpha-Si3N4 powders have been prepared with a yield of 93% by the reaction of Mg2Si with NH4Cl in the temperature range of 450degrees to 600degreesC in an autoclave. X-ray diffraction patterns of the products can be indexed as the alpha-Si3N4 with the lattice constants a = 7.770 and c = 5.627 Angstrom. X-ray photoelectron spectroscopy analysis indicates that the composition of the alpha-Si3N4 samples has a Si:N ratio of 0.756. Transmission electron microscopy images show that the alpha-Si3N4 crystallites prepared at 450degrees, 500degrees, and 550degreesC are particles of about 20, 40, and 70 run in average, respectively.