When rice husk is burnt, about 20wt% of the husk remains as ash. The rice husk ash has more than 95 wt% of silica with high porosity and large surface area, because it still retains the skeleton of cellular structure. These properties of the rice husk ash can be used to synthesize siliceous raw materials such as clay minerals. Syntheses of smectite from the rice husk ash were conducted by using a starting material of a mixture of rice husk ash, magnesium chloride and lithium chloride with a molar ratio of Si: Mg: Li=8: 5.34: 0.66. The optimal conditions of smectite synthesis is as follows. Burning condition of rice husk: 400°C-450°C for 5 hours after smouldered at 350°C for 4 hours, pH value of initial suspension: 13.2-13.5, Reaction temperature: 200°, Reaction time: 1 day-4 days. The rice husk ash as agro-waste can be therefore exploited readily for the hydrothermal synthesis of smectite.