Cross-linkable hyperbranched polymer capped with acryloyl groups (poly-1b) was prepared, and the ionic conductivities and mechanical properties of the composite polymer electrolytes composed of PEO, poly-1b, BaTiO3, and LiN(CF3SO2)2 were investigated in comparison with those of the corresponding composite polymer electrolyte of the non cross-linkable hyperbranched polymer (poly-1a). The cross-linked composite polymer electrolytes of poly-1b showed lower ionic conductivities, but higher tensile strength than the non cross-linked composite polymer electrolyte of poly-1a. This indicates that the cross-linking might improve significantly mechanical property of the composite polymer electrolytes. Addition of tris[2-[2-(2-methoxyethoxy)ethoxy]ethoxy]boroxine (Bx(3)) to the cross-linked composite polymer electrolyte of poly-1b improved ionic conductivity, transport number, and also interfacial stability between a lithium metal electrode and the composite polymer electrolyte.