Synthesis of Mg-alpha-Sialon has been investigated by the mixture of silicon, aluminum and magnesia powders in a flowing nitrogen atmosphere in the range of 1300-1600 degrees C, when Mg-alpha-Sialon is designed with a chemical formulation of Mg (x) Si12-3x Al-3x O (x) N16-x in present work. The results showed that Mg-alpha-sialon initially occurred at 1400 degrees C and basically increased with elevated temperatures. For the samples of x = 0.6, 0.8 and 1.0 the products mainly consisted of Mg-alpha-Sialon with small amounts of Si, AlN and 21R AlN-polytypoid phases at 1600 degrees C. However, in final products of x = 1.2, 1.4 and 1.6 only a little of Mg-alpha-Sialon formed and a great amount of Si remained in these samples at all the fired temperatures. Fortunately, the content of Mg-alpha-Sialon in these samples were obviously increased by adding a small amount of alpha-Si3N4 as seeds before nitridation.