The thermoplastic graft copolymers of chitosan oligosaccharide (PHCSO-g-PCL) were successfully synthesized via ring-opening polymerization (ROP) of ε-caprolactone (CL) through an amino group protection route using phthaloyl chitosan oligosaccharide (PHCSO) as intermediate. The graft reaction was carried out in Pyridine at 120 °C with a chitosan oligosaccharide (CSO) initiator and a tin 2-ethylhexanoate (Sn (Oct)2) catalyst. The prepared copolymer was characterized by FTIR, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), wide-angle X-ray diffraction (WAXD). DSC analysis of PHCSO-g-PCL showed higher melting point at 54.8 °C than linear PCL. The TGA analysis showed that PHCSO-g-PCL was more thermal stable than original CSO. The banded spherulite structure of PHCSO-g-PCL and the growth of spherulite were observed by polarized optical microscope (POM); this was further proven by WAXD results.