A general strategy for the site-specific dual-labeling of proteins for single-molecule fluorescence resonance energy transfer is presented, A genetically encoded unnatural ketone amino acid was labeled with a hydroxylamine-containing fluorophore with high yield (>95%) and specificity. This methodology was used to construct dual-labeled T4 lysozyme variants, allowing the study of T4 lysozyme folding at single-molecule resolution. The presented strategy is anticipated to expand the scope of single-molecule protein structure and function studies.