Cyclocarya paliurus is an excellent ornamental tree with aesthetic form and unique fruit shape. However, the seeds remain deeply dormant for 2 years in their natural environment. To reveal the dormant mechanisms of the seeds, the inhibitor substances were isolated and examined using EMI-MS and spectral analyses. Two compounds were isolated from the pericarp of C. paliurus, and were identified as vanillic acid and protocatechuic acid. Further thin layer chromatography performance showed that the dominant component in C. paliurus pericarp was vanillic acid. The bioassay results indicated that a strong inhibitory effect of vanillic acid existed on embryo germination and root growth of C. paliurus embryo, and the effective concentration required for 50% inhibition (EC₅₀) for embryo germination and root growth were 81.33 mg l^-1 and 35.84 mg l^-1, respectively. The content of vanillic acid in the pericarp of C. paliurus extracted by methanol was quantified up to 0.12 mg g^-1 dry weight. It implied that vanillic acid in the pericarp should be an important germination inhibitor and might play crucial role on seed dormancy of C. paliurus.