Abstract: Castor oil-based polyurethane prepolymer(COPUP) was prepared through the reaction of castor oil(CTO) with 1,6-hexamethylene diisocyanate(HDI), then COPUP was used to modify phenolic foams(PFs). The structure of COPUP was confirmed by FT-IR and 1H NMR. Morphological properties, mechanical properties and thermal stability of the copup-filled PFs were assessed by SEM, universal test and thermogravimetric analysis(TGA). The cell shapes of the COPUP-filled PFs were approximately symmetrical, but the cell sizes were larger than that of pristine PF. Especially, when the cell sizes of PF modified with 10 % COPUP, it became obviously heterogeneous and the bubbles collapse. Compared with that of pristine PFs, the flexural strength of PFs filled with 3%-7% COPUP increased about 21.05%-26.32%; and the corresponding specific flexural strength ranged from 5.90 to 6.17 (kN·m)/kg, which were significantly higher than that of the pristine PFs, 4.96(kN·m)/kg. The addition of COPUP caused the decrease of the thermal stability of PFs.