Hypocrellin B(HB), a peryloquinone derivative, is one of the main photosensitive agents, hypocrellin. This pigment, in combination with phototherapy, has been used in human medicine to cure various skin diseases. The photochemistry (Type I and II) of HB has been studied using spin-trapping and spin-counteraction techniques. The results show that the active oxygen (1O2, O2.-, .OH) mechanism of HB photosensitization can be converted into nonoxygen free radical (HB.-) mechanism completely when oxygen is exhausted in the experimental system. Under anaerobic conditions, HB only undergoes Type I reaction, generating nonoxygen free radical (HB.-).