Abstract: This study aimed to identify the related substances of lorazepam tablets by liquid chromatography mass spectrometry (LC-MS). To separate the related substances of lorazepam tablets, gradient elution was performed using acetonitrile and 0.1% acetic acid -20 mmol/L of ammonium acetate as mobile phase on Inert Sustain C₁₈ (250 mm × 4.6 mm, 5 μm).The accurate mass and elemental composition of the parent ions and their product ions of related substances were determined by electrospray-ionization quadrupole time-of-flight high resolution mass spectrometry (ESI-Q-TOF/MS).The structures of the related substances were identified by spectral analysis. Under the established analytical condition, lorazepam and its related substances were adequately separated, and 22 major related substances with content greater than 0.1% were detected and identified by hyphenated techniques in lorazepam tablets and their stressed samples.Among them, 5 were the impurities listed in the USP or EP, and the others were unknown related substances identified for the first time in this paper.The LC-MS technique can effectively separate and identify the related substances of lorazepam tablets, which provides some reference for quality control.