Silicon was determined by atomic absorption spectrometry with a coated graphite furnace. Pyrolytic graphite tube was coated with tungsten carbide. A mixture of strontium nitrate, ammonium tungstate and EDTA ammonium salt was then added to a sample solution as a matrix modifier. Using a coated tube with tungsten and this matrix modifier, the determination of silicon was successful with high sensitivity and better reproducibility. No interference from metal nitrates and chlorides was observed. In spite of addition of EDTA ammonium salt, the sulfate interference was serious.