Abstract: OBJECTIVE To establish a method for the quantitative determination of DOP in ferrous sulfate sustained release tablets by LC-MS. METHODS Chromatography was carried on a Waters BEH C₁₈ column(50 mm×2.1 mm, 1.7 μm) with the column temperature at 35 ℃, using 95% methanol-5% ammonium acetate water solution(gradiently eluting) as mobile phase at a flow rate of 0.2 mL·min^-1. Determination was performed by multiple reaction monitoring mode in two channels, 391→149, 391→167, using electrospray ionization in positive ion mode. RESULTS DOP had a good linear relationship in 0.0913-0.9130 ng with the linear regression equation Y=268 462X+162 526(r=0.997 3). The recovery of DOP evaluated by determining the spiked samples at low, medium and high concentrations was 98.5%. The limits of quantitation and detection were 0.05 ng and 0.01 ng, respectively. CONCLUSION The method was sensitive and accurate which could be an appropriate technique to detect the DOP in ferrous sulfate sustained release tablets.