A precursor ion scanning method in liquid chromatography/tandem mass spectrometry (LC/MS/MS) has been developed to identify metabolites of 4-hydroxybenzoic acid esters (parabens) in urine as a first step of paraben-metabolites determination in urine. The identification was confirmed by analyzing elemental compositions of conjugation group ions and those fragment ions with LC/Q-ToF MS/MS. Two milliliters of urine diluted with the same volume of acetonitrile was passed through an ODS cartridge column. After the urine sample passed through the ODS cartridge, the cartridge was added with 6mL of methanol elute, which was concentrated to 4mL. The concentrate was filtrated with a polytetrafluoroethylene (PTFE) filter of 0.45 mm pore size. Then the filtrates with PTFE was prepared to a sample for LC/MS/MS and LC/Q-ToF/MS after being concentrated to 1mL. Precursor ion scan by monitoring product ions of [M-H]－ of paraben molecules was applied for finding paraben conjugates. Precursor ions of m/z245 and 341 were found as candidate ions that produce [M-H]－ ion of ethyl-4-hydroxybenzoate (ethyl paraben). In order to confirm elemental compositions of the candidates, accurate masses of the m/z245 and 341 were respectively determined to be m/z245.0138 and 341.0875 in the mass error within 1.5ppm by LC/MS/MS. The elemental compositions of C9H9SO6 and C15H17O9 were respectively confirmed from the accurate masses of m/z245.0138 and 341.0875 within mass error of 1.5ppm. If the ions of C9H9SO6－ and C15H17O9－ are conjugates of ethylparaben, those would be respectively produced by esterization with chemicals of H2SO4 and C6H10O7 in elemental composition. From the compositions of H2SO4 and C6H10O7, sulfuric acid and glucuronic acid were retrieved as a top scored chemicals in a database(ChemSpider). The retrieved chemicals are well known as conjugating chemicals to parabens, and the retention times of the C9H9SO6 and C15H17O9 peaks were respectively agreed with those estimated from the logKow values of ethylparaben conjugates of sulfuric acid and glucuronic acid. Then we concluded that the ions of C9H9SO6－ and C15H17O9－ were [M-H]－ of ethylparaben conjugates of sulfuric acid and glucuronic acid, respectively. With the similar manner, some of other conjugating chemicals were estimated.