The decatungstate photocatalyst [W10O32]4- efficiently promoted the C(sp3)-H alkylation of the trifluoroacetic acid salt of valine methyl ester (H-Val-OMe·TFA) with electron-deficient alkenes under UV irradiation. The electrostatic interaction between the cationic ammonium group (+NH3) of the main chain and anionic [W10O32]4- played an important role in this reaction. The influence of various protected amino acids in the C(sp3)-H alkylation was investigated as the model reaction for the alkylation of Val-containing peptides. The introduction of an alkyne moiety into Val through this alkylation was successful, and successive copper-catalyzed azide-alkyne cycloaddition (CuAAC) was demonstrated. The C(sp3)-H bond of a Val residue located at the second from the N-terminus was also successfully converted. C(sp3)-H alkylation of oligopeptides containing two Val residues selectively proceeded proximally to the N-terminus.