An on-line solid-phase extraction-liquid chromatography-tandem mass spectrometry (on-line SPE-HPLC-MS/MS) method was developed for the analysis of metabolites of three phthalate esters in human urine at the low nanogram per milliliter level. The recoveries were above 84.3% and relative standard deviations varied from 0.8 to 4.8%. The compounds along with their deuterated internal standards were detected in the negative ion mode by selective reaction monitoring and the accuracy of the method was improved by isotope dilution. Monobutyl phthalate was detected with median level of 22.5 ng/ml. The median levels for monobenzyl phthalate and monoethylhexyl phthalate were less than the limit of quantitation (LOQ). The on-line SPE-HPLC-MS/MS method allowed the possibility of determining these metabolites within a short time, with increased sensitivity and by using decreased amounts of sample and solvent.