Four impurities were isolated from raw material of clindamycin phosphate (CP), and their structures have been determined. LC-MS was used to determine the molecular weights of the impurities in the raw material of CP. Reversed-phase preparative HPLC was used to prepare them, and their chemical structures were identified by HR-MS and NMR. The four unknown impurities were determined as clindamycin-B-phosphate (1), clindamycin-2,4-diphosphate (2), 3',6'-dehydro clindamycin phosphate (3), epi-clindamycin phosphate (4). Impurity 1 has been included in BP and EP, while 2, 3 and 4 have not. The impurities 2, 3, 4 are first separated from raw material of CP.