Formation of the methyl- and n-butyl monolayer on (Cl)Si(I 11) surfaces by using the Grignard reaction and their electro-chemical behavior was investigated. The modified Si(I 11) surfaces were indicated to be closely packed with a single moiety species. The methyl-modified Si(I 11) surface was composed of (I x 1) adlattice structure. The methyl-modified surface was stable in aqueous solution, and showed essentially good electrical conductive property with p-n junction like behavior.