Introduction CD spectra provide information on the secondary structure of proteins and the environment of aromatic side chains. Therefore, CD measurement using a stopped-flow system is considered as one of the best methods for analyzing the unfolding and refolding of proteins. The existence of an intermediate between denaturated state and natural state during the refolding of proteins has been reported. The CD stopped-flow method is used for examining this refolding process. In this report the refolding process of cytochrome c (cyt c) measured using a SFS-492 stopped-flow system will be explained. Keywords: Stopped-flow, Circular Dichroism, Refolding Sample Preparation Aqueous solution of Cytochrome c denaturated by guanidine hydrochloride (GuHCl) was diluted with 0.1 M acetic acid buffer solution (1:9). The refolding process was observed at 222 nm for the secondary JASCO圓二色光譜儀CD J-1500