The main purpose of this paper was to find out the background of Ŭmnu’s pludering of North Okchŏ. For this, it analyzed Ŭmnu people’s ethnicity and bellicosity described in “Biographies of Dongyi (東夷傳)” of Sanguozhi, and compared them with archaeological evidences. According to these records, the plundering could be characterized as periodical and had economic purpose, which indicates a driver within Ŭmnu’s society that turned the people into looters around the third century. Furthermore, in regard to this topic, the paper also sought the process that formed the ethnic recognition of Ŭmnu in the Biography. It suggested that the ethnicity in the book was the mixture of the information from Wei's expeditionary force, who contacted its tribes in coastal region of East Sea in the middle of the 3rd century, and the perception of Puyŏ, who had subjugated and governed Ŭmnu for a long time.
Also, this paper examined the correspondence between the records about Ŭmnu’s plundering and the characteristics of various cultural groups in this area through archaeological data. Accordingly, it can be said that the Poltse culture of Maritime Province was the most correspond to the culture of the looters who plundered North Okchŏ. This periodical pillages from Ŭmnu was strongly related to the widespread social change within the Poltse culture around the third century. During this period, there was armed struggles among the tribes of Ŭmnu, while their traditional trade route via Puyŏ was diminished after they broke away from their neighbor faction’s subjugation. As a result, large-scale polities such as the polity in Fenglin culture emerged. However, on the other hand, while it forced some tribes that lost in the competition to take refuges to the other areas, this armed competition induced the other tribes to plundering the neighbouring cultures.