Acylated derivatives of chitosan oligomers with a relatively high degree of polymerization (DP=4, 5, or 6) were synthesized and their mesophase properties were studied in comparison with those of the lower-DP homologues (DP=2 and 3) previously reported. By differential scanning calorimetry (DSC), all the compounds were found to show thermotropic mesomorphism in a temperature region below 150°C. The isotropization temperatures decreased with increasing DP, while the melting point remained at around 65°C, independent of DP. In addition to the narrowing of the mesomorphic region, the optical anisotropy as observed under a polarizing microscope remarkably decreased with increasing DP, indicating that the mesophase becomes less stable and ordered with an increase in DP. An X-ray diffraction analysis revealed that the tetramer as well as the dimer and trimer form a hexagonal ordered columnar phase (Dho), while the pentamer forms a disordered one (Dhd). On the other hand, the phase formed by the hexamer was not a columnar but a discotic nematic phase (ND). This is attributable to the highly anisotropic shape of the oligosaccharide core in the hexamer.