Objective To determine dissociation constants of norathyriol by UV spectrophotometry. Methods Norathyriol was dissolved in phosphate buffer solution of different pH values， and the pKa values were calculated according to the measured absorbance of the solutions. Results The pKa value was found to be 6. 72 which the point of inflection using mapping software fitting curves using wavelengths 253 and 315 nm for the determination respectively. Conclusion The dissociation constant of norathyriol was determined using ultraviolet spectrometry. It has important reference significance for the investigations of absorption， distribution， metabolism and excretion of the drug. This method is simple， rapid， accurate and reliable.