A powdered natural clinoptilolite-rich tuff (100 mesh) from Semnan region of [ran was successfully converted to higher value-added synthetic zeolite A (submicron particles of similar to 200-300 nm) in a single stage procedure under a hydrothermal condition. Effect of different parameters such as crystallization temperature, sodium hydroxide concentration, solid/liquid ratio (the mass of clinoptilolite: g; to the volume of sodium hydroxide solution; ml) and crystallization duration on the final product were investigated. The formation of zeolite A was ascertained by X-ray powder diffraction, X-ray florescence (chemical analysis), and scanning electron microscopy techniques. The results were revealed that reaction temperature of 353 K, sodium hydroxide concentration of 2 M, crystallization time of 4 h, and solid to liquid ratio of 1/18 were optimized condition for this conversion. (C) 2009 Elsevier B.V. All rights reserved.