First, the fluorescence-reactions between glucuronic acid (D-glucuronolactone) as an uronic acid andN-benzyl-2-naphthylamine(NBNA) orN-benzyl-1-naphthylamine(NB-1-NA) as a secondary naphthylamine were systematically investigated in acetic acid medium. Secondly, NBNA-glucuronic acid fluorescence-product that has a maximum emission wavelength(Em) at 508 nm with excitation wavelength(Ex) at 468 nm was used for a selective assay of glucuronic acid; the calibration curve was linear in the range from 5 ng/ml to 2.0 μg/ml glucuronic acid, and the recovery test in normal urine was satisfactory (98.4–102.5%).