Abstract: Herein disclosed are a tert-butyl cycloalkyl dialkoxysilane compound which is quite useful as a water repellant for treating the surface of a variety of construction materials such as wood, concrete and marble and which can easily be used, and a method for preparing the compound. A Grignard reagent represented by the formula: R.sup.1 MgX is reacted with a silane compound represented by the formula: (CH.sub.3).sub.3 CSiH(OR.sup.2).sub.2. Then the resulting silane compound: (CH.sub.3).sub.3 CSiHR.sup.1 (OR.sup.2) is further reacted with an alcohol: R.sup.2 OH in the presence of a catalyst to give a tert-butyl cycloalkyl dialkoxysilane compound: (CH.sub.3).sub.3 CSiR.sup.1 (OR.sup.2).sub.2.