Modification of protein thiol is one of the most important post-translational modifications and it occurs depending on the redox state in cells. Protein S-nitrosylation is NO (nitric oxide)-dependent modification of protein thiols and is crucial for regulation of cellular functions such as transcription, protein expression, and signal transduction. Maleimide reagents are generally used to assess the redox status of the thiols in a protein of interest. The maleimides AMS and polyethylene glycol-maleimide (PEG-Mal) have generally been used to distinguish between the reduced and oxidized states of proteins. We have introduced a photocleavable group between the PEG and the maleimide moiety and designated these molecules as PEG-PCMal. When a PEG-PCMal-labeled protein is separated by SDS-PAGE and subsequently irradiated with UV on the polyacrylamide gel, the PEG moiety is removed from the protein. In this study, we tried analysis of protein S-nitrosylation using a new maleimide reagent PEG-PCMal.