This article figures out the original Characters of Sino-Korean words “Seo-sil” and “*Cheo”. “Seo-sil” is usually written as “閪失” in historical records of Chosen dynasty, and means “lost by carelessness”. “*Cheo” is a root of words which means “first” or “start” in Korean. According to the meaning of each character, historical records and the phonological correspondence, this article confirms that the original Chinese characters of “Seo-sil” and “*Cheo” are “疏失” and “初”.