A tert-hydrocarbyl, e.g. tert-butyl, silyl compound can be synthesized easily according to the method of the invention in which a tert-hydrocarbylmagnesium halide as a Grignard reagent is reacted with a silane compound having at least one silicon-bonded hydrogen atom and at least one silicon-bonded halogen atom simultaneously in a molecule in a suitable organic solvent so that the halogen atom in the latter reactant is replaced with the tert-hydrocarbyl group in the former reactant to give the desired tert-hydrocarbyl silyl compound in a high yield without the safety problem in the conventional method using a tert-alkyl lithium as the reactant.