A new Si surface modification with organic groups via hydrosilylation at room temperature is reported. A hydrogen-terminated Si (111) surface was reacted with activated alkynes such as propiolate esters, propiolic acid, but-3-yne-2-one, propiolonitrile and phenylacetylene at room temperature for 24–40 h to give modified surfaces. The coverage ratio was estimated to be 31–56% by XPS analysis. This surface modification method is mild, highly efficient and procedurally simple. The novel surfaces modified with various organic functional groups are expected to have interesting utilities.