Tetraphenylporphyrinato methylphenylsilicon complex (abbreviated to Si(TPP)MePh) was synthesized and structurally characterized. Although the Si-Ph bond of Si(TPP)MePh is much longer than the Si-Me bond, photolysis and chemical oxidation of Si(TPP)MePh cleaved the Si-Me bond selectively and gave new phenyl-substituted silicon porphyrins. Theoretical studies of a model compound showed that significant sigma-pi orbital interaction raised the HOMO and is responsible for the silicon-carbon bond cleavage. The instability of phenyl radical to methyl radical caused selective bond cleavage.