Abstract: It was found that, at pH 11.02-11.39, the characteristic absorption peak of octanohydroxamic acid in UV adsorption spectra appeared at 216 nm. Based on this fact, UV spectrophotometry was developed for determination of trace octanohydroxamic acid in water. In Na2HPO4-NaOH buffer solution (pH 11.2), linear relationship was found between the absorbance at 216 nm with the concentration of octanohydroxamic acid ranging from 1.0×10-5 to 2.1×10-4mol·L-1, with detection limit (3s) of 1.4×10-6mol·L-1. The molar absorption coefficient was 6.613×103L·mol-1·cm-1, which was 10 times of the molar absorption coefficient of iron (Ⅲ)-octanohydroxamic acid complex at 535 nm in visible spectrophotometry. The proposed method was used to determine two octanohydroxamic acid samples, giving RSDs (n=5) less than 2.0%.