Abstract: The study aims to develop a rapid, specific and sensitive method for quantitative analysis of trace impurities in levofloxacin formulation using LC-MS/MS. The quality of the different formulations from 19 plants was evaluated in the contents of the impurities. The results indicated that there were 5 impurities in the samples, and the content was different in the products with same formulation by different plants. The products of 3 plants were in good quality with impurities level under 0.01%. Levofloxacin N_4'-methyl quaternary impurity was first reported as the formulation impurity. The impurities were tightly correlated to the reservation of drug, process control of formulation and storage during transportation. The results suggest that our method is sensitive and specific to detect the trace impurities in formulation, and can be used to monitor the quality of commercial drug product.