Human hepatocarcinoma cells, line 7703, suspended in PBS were incubated with hematoporphyrin (HP) at 37℃ for an hour and then irradiated by 340—370 nm UV for 32 minutes (~3.9×10,-2, J/cm•,2,min) at 4℃.The Fornace and Kohn's method of alkaline elution combined with protease treatment was used for measuring the SSB and crosslink of cell DNA. Three results were obtained: (1) Cell DNA degradation occurred in incubating the cells with HP (3.1—50.0 µg/ml) for an hour. (2) The HP (50 µg/ml) photosensitization enhanced the cell DNA crosslinking by a factor of 2. (3) About 87% of the DNA in crosslinked substance induced by UV in the presence of HP is linked with protein. The enhancement of DNA crosslinking by HP photosensitization reported here has not been found in previous papers so the authors suggest that this phenomenon shoud not be overlooked in considering the mechanisms of cell killing in photoradiation therapy of cancer using HP as the photosensitizer.