Abstract: To identify the related substances of tacrolimus by LC-MS techniques, the separation of the related substances contained in tacrolimus and its stressed samples was carried out on a Agilent Eclipse Plus-C₁₈ (150 mm × 4.6 mm,3.5 μm) column with 0.01% fomic acid solution and a mixture of acetonitrile- methyl tert-butyl ether as the mobile phase.in linear gradient elution. Electrospray positive ionization high resolution Q-TOF/MS was used for the determination of the accurate mass and elemental composition of the parent ions of all the components, thence, the structures of all the detected substances were successfully characterized through spectra elucidation and fragmentation pathways analysis. Tacrolimus and its 35 detected related substances were well separated under the established conditions, among which 2 were its isomers , 3 were listed in United States Pharmacopeia(USP), and the rest 30 were identified as unknown products having not been reported before. These results were useful for its fermentation processes and quality control.