Abstract: A method of ultra flow liquid chromatography-tandem mass spectrometry (UFLC-MS/MS) was developed to elucidate the impurity of linezolid tablets. Linezolid was subjected to forced degradation under hydrolytic (acid, base and neutral), oxidative, photolytic and thermal. The structure identification of the degra­dation products and the fragmentation patterns for the related impurities were analyzed. A total of four degra­dation impurities were characterized, impurity 1 is (S)-1-amino-3-((3-fluoro-4-morpholinophenyl)amino)propan-2-ol, impurity 2 is (S)-4-(4-(5-(acetamidomethyl)-2-oxo-oxazolidin-3-yl)-2-fluorophenyl)morpholine 4-oxide, impurity 3 is (S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinophenyl)oxazolidin-2-one, impurity 4 is (R)-N-(3-((3-fluoro-4-morpholinophenyl)amino)-2-hydroxypropyl)acetamide. Acid degradation induced impurity 3 and impurity 4, base degradation induced impurity 1 and impurity 4, oxidation degradation induced impurity 2, hydrolysis degradation induced impurity 4. The study also determined calibration factor using impurity references, and the calibration factors were found to be 1.3, 1.4, 0.9 and 1.1, respectively. The toxicity of the degradation impurities was predicted by web-based prediction system. The results from this study provide an important reference in quality control and evaluation of linezolid.