Abstract: The aim of this paper was to study the species and sources of related substances in montelukast sodium granules based on HPLC-MS/MS. The related substances of the already marketed montelukast sodium granules(Singulair, 4 mg)were firstly separated and identified by HPLC-MS/MS method, and their chemical structures and sources were confirmed according to the pulished synthesis process and degradation studies. The results revealed that there were 7 related substances, whose chemical structures and sources were confirmed based on the MS/MS technique and the related literature. Impurity 7 , montelukast quinoline ring oxynitride, had not been reported. This study systematically analyzed the related substances in the commercical product, which provides useful information for the development and quality control of the product.