Abstract: The micro-level DINP in polyester plastics was studied by normal phase high performance liquid chromatography-electrospray ionization mass spectrometry (NP-HPLC/ESI MS). The results show that this method is reliable and ensured the accurate determination of DINP in polyester plastics. The linearity of the curves is very good with the correlation coefficient of 0.999 4. The limit of detection (LOD) is 0.05 mg/L. The recoveries are between 95% and 103%. This method can provide a reliable analytical tools and technical assurance for the quality control of this kind of polyester plastics.