Phlorotannins are polymers of phloroglucinol known only from brown algae with various bioactivities.But studies on biosynthesis pathway of phlorotannins have rarely been reported. Studies on the autoxidation process of phloroglucinol revealed that polymerization occurred during autoxidation and that the ultraviolet spectrum of polymers was similar to that of phlorotannins. High Performance Liquid Chromatography analysis indicated that molecular weight distribution of polymers was diverse and similar to natural analogue.