A novel photosensitizer, pentalysine β-carbonyl-phthalocyanine zinc [ZnPc-(Lys)5] has tendency to form aggregate in aqueous solution. The observed in vivo Photodynamic therapy (PDT) effect of ZnPc-(Lys)5 suggests a disaggregation mechanism. In this study, the equilibrium binding constant Ka, the numbers of binding sites n and the distance of Forster radii r between ZnPc-(Lys)5 and human serum albumin (HSA) are measured by Spectroscopy. A molecular model of HSA-ZnPc-(Lys)5 complex was generated according to these datum. This molecular model provides rationale that the molecular interaction between HSA and ZnPc-(Lys)5 facilitates the dissociation of ZnPc aggregates.