Chlorella T‐1 is a micro algae which carries out highly active photosynthetic carbon fixation. For searching the ·OH scavenging molecules in this strain, the methanol extract of T‐1 was fractionated by RP‐HPLC and the fractions were analyzed for DMPO‐OH formation by ESR after Fenton reaction. In the course of study, a single chlorella T‐1 ingredient was found to enhance the formation of DMPO‐OH rather than scavenge in the Fenton system. The ·OH enhancing chlorella component was further purified by chromatography and finally identified as lactic acid by NMR and LC‐MS. From the analysis of the reaction conditions for DMPO‐OH, it was found that lactic acid did not serve as direct source of ·OH but accelerated Fenton reaction.