Photodynamic effects of HPD has been used for cancer therapy (PDT) successfuly. The biological mechanism of the PDT was studied by using DNA as a target in this work. After treatment of phage M13 DNA with HPD + light the molecule of the DNA kept intact in TE buffer. But the smear bands by puting DNA in 0.1 mol/L NaOH at 90 degrees C and separating with electrophoresis denoted the degradation of DNA at the alkali-labile sites of phosphodiester bond in which the groups of the bases were photooxydized. The primer extension experiment showed the aggregation of DNA template by photooxydation. The DNA sequencing results showed that some of the wrong sequences appeared both at the site of A,G,C,T bases. The mouse Ehrlich ascetes carcinoma cell DNA was treated as above then degraded by Bgl II and BamH I respectively. It was found that the restriction fragments became larger than the controls. It also meant the alteration of the bases of DNA by the photooxydation. So the four bases of A,G,C,T of DNA were altered by photodynamic effects of HPD.