The aggregation characteristics of thionine(Th)-sodium dodecylsulfate(SDS) and Th-SDS-chitosan system were investigated by fluorescence spectrophotometer. The fluorescence intensity of Th-SDS-chitosan system at S/D(the concentration of SDS to Th ratio)<10 decreased gradually in the order of 16.0 10⁴<8.0 10⁴4.0 10⁴, molecular weight of chitosan. When the concentration of organic solvent was below 30%(v/v), with increasing in molecular weight of chitosan the fluorescence intensity of Th-SDS-chitosan systems increased higher than that of Th-SDS system.