Abstract: Nanocomposites of waterborne polyurethane (WPU) with functionalized graphene nanosheets (FGNs) were prepared by direct solution blending. The isothermal crystallization behavior of the nanocomposites was investigated by differential scanning calorimeter (DSC), and the isothermal crystallization kinetics was analyzed by Avrami equation and Arrhenius equation. Results showed that Avrami equation was valid for analyzing the isothermal crystallization behavior of WPU-FGNs nanocomposites generally. With the crystallization temperature increased, the crystallization half-time (t_1/2) and crystallinity of the composites were both increased. At 287 K, crystallization enthalpy of the composites increased from -16.88 J/g to -35.38 J/g when the mass fraction of FGNs increased from 0.1% to 1.0%. Crystallization activation energy of WPU was -0.74 kJ/mol. When the mass fraction of FGNs was 0.1%, crystallization activation energy of the composite was -1.25 kJ/mol. When the mass fraction of FGNs was 1.0%, crystallization activation energy of the composite was -0.83 kJ/mol.