Abstract: PU/nano－CaCO₃ composite materials were obtained by step reaction polymerization. The authors researched the influence of nano－CaCO₃ contention on the mechanical and thermal properties of composite materials. It indicated that with the increase of nano－CaCO₃ contention, the mechanical properties of composites, such as the tensile strength, elongation at break, etc. and thermal properties were increased. From the comprehensive data of all experiments, it showed that when the nano－CaCO₃ content was 6%, the mechanical comprehensive property and thermal property of composite materials were the best.