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To produce Na₂SnO_3, which is widely used in the ceramics and electroplating industries, a novel process for the preparation of sodium stannate from cassiterite concentrates was developed successfully by the authors’ group. It was found that sodium stannosilicate (Na₈SnSi₆O₁₈) was easily formed due to the main gangue of quartz in cassiterite concentrates, which was almost insoluble and decreased the quality of Na₂SnO₃. The formation and transitions of Na₈SnSi₆O₁₈ in the SnO₂–SiO₂–Na₂CO₃ system roasted under a CO–CO₂ atmosphere were determined. The results indicated that the formation of Na₈SnSi₆O₁₈ could be divided into two steps: SnO₂ reacted with Na₂CO₃ to form Na₂SnO₃, and then Na₂SnO₃ was rapidly combined with SiO₂ and Na₂CO₃ to form low melting point Na₈SnSi₆O₁₈. In addition, Na₈SnSi₆O₁₈ can be decomposed into Na₂SiO₃ and Na₂SnO₃ by using excess Na₂CO₃.