The present invention relates to a novel crystalline form of 5-chloro-N-({(5S)-2-oxo-3-[4-(5,6-dihydro-4H-[1,2,4]triazin-1-yl)phenyl]-1,3-oxazolidin-5-yl}methyl)thiophene-2-carboxamide methanesulfonate and a pharmaceutical composition containing the same. The novel crystalline form of a compound according to the present invention exhibits excellent stability even in high-temperature and humidity environments, and thus can be favorably used to prevent or treat diseases, such as thrombosis, myocardial infarction, atherosclerosis, inflammation, stroke, angina pectoris, restenosis after angioplasty, and thromboembolism.