Abstract: RP-HPLC was applied to the determination of dilauryl thiodipropionate (DLTDP), a commonly used anti-oxidant. Symmetry C18 column was used as stationary phase and methanol-H2O mixture (98+2) was used as mobile phase in the separation. Differential refractivity detector and external standard method were adopted in the determination. Linear relationship between values of peak area and mass concentration of DLTDP was obtained in the range from 0.20 to 20.0 g·L-1 with its lower limit of determination (10S/N) 0.10 g·L-1. Samples of the anti-oxidant DLTDP from 5 batches were analyzed by the present method, mass fraction (w%) of DLTDP found were ranged from 95.6% to 99.3% with RSD′s (n=5) less than 1.1%.