Ulleungdo is first mentioned in historical documents in 1145 A.D., when it is mentioned by Kim Bu-sik (김부식) in Samguksagi (삼국사기), a Goryeo text describing the histories of the countries of Silla (신라), Goguryeo (고구려), and Baekjae (백제). Here is the relevant passage from the text:
十三年 夏六月 于山國歸服 歲以土宜爲貢 于山國在溟州正東海島 或名鬱陵島 地方一百里Ulleungdo is also mentioned in the Samgukyusa (삼국유사), another Goryeo text written around 1277 A.D. Here is the relevant passage:
13년(512) 여름 6월에 우산국(于山國)이 항복하여 해마다 토산물을 바쳤다. 우산국은 명주(溟州)의 정동쪽 바다에 있는 섬으로 혹은 울릉도(鬱陵島)라고도 한다. 땅은 사방 100리인데....
In the summer month of June in the 13th year (512 A.D.), Usan-guk surrendered and began paying tributes in local products. Usan-guk is a island in the sea due east of Myeongju, and is also called Ulleungdo. It had an area of 100 "ri."
又阿瑟羅州(今溟州) 東海中便風二日程 有于陵島(今作羽陵) 周廻二萬六千七百三十步
하슬라주(지금의 명주)는 동쪽 바다 가운데 바람이좋으면 2일 거리에 우릉도(지금의 우릉)이 있다. 주위는 2만6천7백3십보이다.
Ulleungdo (now U-leung) is in the middle of the sea, two days due east of Haseullaju (now Myeongju) if the wind is favorable. It has a circumference of 26,730 "보."
Notice that the passage says that Ulleungdo is two days sailing from Myeongju (now Kangneung) and that it has a circumference of 26,730 bo, which is only about 32 kilometers.Koreans usually say that the Usan-guk mentioned in the Samguksagi was the first referrence to Dokdo/Takeshima, but as the text clearly says, Usan-guk was just another name for Ulleungdo. In fact, the Chinese characters used for the names "Usan" and "Ulleung" seem to have essentially the same meaning.
The Chinese characters used for Usan-guk (于山國) literally mean "Big Mountain Country." 于 (우) means "big," 山 (산) means "mountain," and 國 (국) means "country." The Chinese characters used for Ulleungdo (鬱陵島) literally mean "Luxuriant, Big-hill Island." 鬱 (울) means "luxuriant," 陵 (릉) means "big hill," and 島 (도) means island. Notice that Usan refers to a "mountain" and Ulleung refers to a "big hill." Moreover, notice that in the Samgukyusa, the 鬱 (울) in Ulleungdo was replaced with 于 (우), the same character used in Usan-guk. Therefore, not only does the Samguksagi specifically say that Usan-guk and Ulleungdo are the same island, the name "Ulleung" seems to be just a linguistic variant of "Usan."
So why do Koreans say that Usan-guk is a reference to Dokdo/Takeshima? Well, they say that since the "guk" in Usan-guk means "country," then that means it would have included neighboring islands, including a small group of rocky islets ninety-two kilometers away, but there is no evidence in either of the documents mentioned above that would support that claim. In fact, Ulleungdo was the only island mentioned. Moreover, the 100-"ri" measurement of the land mentioned in the Samguksagi and the circumference given in the Samgukyusa are further evidence that the documents were referring to only Ulleungdo. Besides, why would a people name their country after a small group of barren rocks 92 kilometers away instead of after the island they actually live on?
To look at only the documents above and say that Usan-guk is referring to Dokdo requires a leap with a triple back flip and a double twist in logic.