Disclosed herein are a silane coupling agent and a method of preparing the same useful for a composite organic-inorganic material. The silane coupling agent is represented by the formula of R1R2R3Si—X, wherein R1, R2, and R3, respectively, represent straight or branched alkyl having 4 to 22 carbon atoms, alkoxy, phenyl, phenyl alkoxy, benzyloxy or phenyl alkyl group. The method comprises the steps of: dissolving a vinyl derivative in a solvent under a nitrogen or argon atmosphere; and reacting the resulting solution with an alkoxysilane derivative at a temperature of 20 to 200° C. for 1 to 72 hours in the presence of a metal catalyst. Alternatively, the method comprises the steps of: dissolving alkylmagnesium halide in a solvent under a nitrogen or argon; and reacting the resulting solution with haloalkoxysilane at a temperature of −78° C. to 50° C. for 0.1 to 5 hours.