N-phthalolyl chitosan with the degree of substitution 1.0 was prepared from completely deacetylated chitosan．Its chemical structure was characterized by means of FTIR and 1H-NMR．This polymer was used to investigate the thermotropic phase transition of lytropic liquid crystalline solutions in dimethylsulfoxide (DMSO) by using differential scanning calorimetry (DSC)．The critical concentration of mesophase formation was 43 wt％determined both by DSC and polarizing light microscopy．An evident gel-sol transition in addition to a liquid crystal-isotropic liquid transition (the clearing transition) was observed by means of DSC in solutions whose concentration was higher than 43 wt％. It means that the isotropic liquid transferred into anisotropic sol and then into anisotropic gel with the decrease of temperature. The gel-sol transition temperatures and transition heats of N-phthalolyl chitosan／DMSO solutions were much higher than those of random substituted N-phthalolyl-O-acetyl chitosan solutions in some solvents reported previously．It may imply that the regularity of substitution is very important for the formation of anisotropic gel．A phase diagram of temperature and polymer weight fraction was presented according to the DSC results for the N-phthalolyl chitosan／DMSO system．