This paper argues that Koguryo historically belonged to Korea based on the linguistic homogeneity among Koguryo, Sinra, and Paekche. The language of Koguryo shows the following four distinct properties from Chinese. First, Koguryo, Sinra, and Kara share a dual structure of national birth myths, and the directional vocabulary found in the countries also shows the evidence that the Korean people moved from North to South. Chinese, on the other hand, has no indication of these facts. Second, the place names in Koguryo shared the same properties with those of the countries in the Korean Peninsula at the same period, but were different from those of China. It is also noticeable that the national name of Korea, adopted from the national name of Koruryo, was announced to the world and that Korea has been acknowledged as our national name since then. The legitimate lineage of Koguryo > Parhay > Hu-Koguryo > Korea proves that Koguryo belonged to Korea historically, Third, the names of kings were very similar among the Three Kingdoms, but distinct from those of China. The names of kings were created on the basis of the sun as well as nature and human functions or activities like nuri (world), Piryu (originated from the name of a river), onjo (all over world), Chumong Hwar-bo (a person who is good at shooting), Paem-bo (a baby crawling like a snake), Kulcornerchir-bo, Isa-bo, etc. This is a common characteristic among the Three Kingdoms, but clearly different from China. Fourth, the governmental names were very similar among the Three Kingdoms, but distinct from Chinese ones. These linguistic properties shared among the Koguryo, Packche, and Sinra are never accidental, but provide reliable evidence that the countries were founded by the Korean people. Koguryo, therefore, belonged to Korea historically.