The　sulfonated　phthalimidomethyl　phthalocyanine　Zinc　(ZnPcSP)　is　an　amphiphilic　photosensitizer　against　cancer.　The　characteristic　absorption　peak　of　monomer　and　aggregate　of　ZnPcSP　were　observed,　and　the　effects　of　solution　compositions　on　its　aggregation　were　investigated.　In　70%　MeOH/H2O,　70%　EtOH/H2O,　50%　EtOH/H2O,　70%　i-PrOH/H2O,　70%　CH2OHCHOHCH3/H2O,　50%　DMSO/　H2O,　50%　DMF/H2O(volume　fraction)　and　CEL　solutions,　ZnPcSP　existed　in　the　equilibrium　between　monomer　and　dimer,　the　dimerization　constants　were　1.　314　×　105,　8.　496　×　104,　9.　753　×　104,　5.　583　×　104,　3.　809　×　105,　4.　430　×　103,　8.　679　×　105　mol/L,　respectively.　But　trimerization　of　ZnPcSP　took　place　in　water　and　saline　solution,　and　trimerization　constants　were　1.　296×108　and　2.　236×108　mol-2　·　L2　respectively.　Furthermore,　the　effect　of　existence　states　of　ZnPcSP　on　the　activities　against　S180　carcinoma　were　estimated.　The　result　showed　that　the　inhibitory　rate　of　ZnPcSP(3.　1×10-6　mol/L)　in　CEL　solution　mainly　existing　in　the　form　of　monomer　was　about　8　times　higher　than　that　of　ZnPcSP　in　saline　solution　mainly　existing　in　the　form　of　trimer.