This paper examines the dating and characteristics of the clay seals excavated from Lelang fortress in Pyeongyang of North Korea. This fortress has estimated where the office of Lelang commandery had located as many artefacts of Han China had been discovered. Some archaeologists insisted that the office of Lelang commandery established at the fortress in 108 BC, but some historians thought that the office was moved from Wanggeomseong of the capital of Old Joseon at the north of Daedong river to the fortress at south of Daedong river after a certain period from 108 BC. In this regard, the views of authenticity of the clay seals from the fortress, which first appeared in 1919 and rapidly increased in the number of cases of discovery, were confronted. This article have examined the process of discovery and excavation of Lelang fortress since 1913 and the authenticity of clay seals from Lelang fortress. Next, I estimated the dating of clay seals through the inscriptions and the type of the clay seals from Lelang fortress. For this, I compared the inscriptions of the eight official clay seals excavated by Tokyo Imperial University in 1935 and 1937 to the writing the Wood tablet of Census of Lelang commandery in 45 B.C. discovered in Jeongbaek-dong tomb no. 364 of Pyeongyang in early 1990s. Finally, the usage of clay seals was deduced by the distribution of clay seals at the site and the type classification by use trace of the clay seals. I searched for the possibility of dating on the Lelang fortress based on the dating and characteristics of the clay seals.