A new method of determination of propyl gallate(PG) in cooking oil by spectrophotometry was developed, based on the principle of propyl gallate(PG) was reduced by chloroauric acid to produce gold nanoparticles, which had a strong absorption peak at 530nm. The effects of chloroauric acid concentration, heating temperature and reaction time on the systems, and interference test of coexisting substances in cooking oil were investigated, the conditions were optimized. Under the optimum experimental conditions, the concentration of PG was proportional to the △A in the range of 0.34～13.65 μg/mL. The linear equation was △A530nm=0.160 3ρ+0.056 1, with correlation coefficient was 0.994 6, and the detection limit was 0.093 μg/mL. The result of determine PG in cooking oil by the method was the RSD 0.9%～3.1%, and the average recovery rate 90.3%～97.0%.