Chemical constituents of Uncaria hooks were studied. Four compounds were isolated from the ether soluble fraction by chromatographic purification process. They were ident ified as isocorynoxeine-N-oxide, anti-isorhynchophylline-N-oxide, physcion and ursolic acid by spectral evidences. Their contents analyzed by HPLC or GC method were physcion 0.04%, ursolic acid 0.26%, isocorynoxeine-N-oxide 0.04% and anti-isorhynchophylline-N-oxide 0.03% respectively. Isocorynoxeine-N-oxide was not reported in nature so far.