In the Qing period, Outer Mongolia was administered under the Čiγulγanqosiγu system or League-banner system, and borders of various ayimaγ and qosiγu were established, as a part of this system. The task of establishing the borders of ayimaγ and qosiγu began in earnest in the 46th year of the reign of Qian Long 乾隆 (1781), because the borders of the western three ayimaγ of Outer Mongolia, ǰasaγtu Qan Ayimaγ, Sayin Noyan Ayimaγ and Tüsiyetü Qan Ayimaγ, were demarcated in this year by Batu. Batu was delegated by the Qian Long Emperor with the mission to settle the pasture conflicts among these three ayimaγ, which had occurred with the westward expansion of pastureland in Outer Mongolia after the fall of ǰegün γar, the Dzungars. But, in the case of the eastern two ayimaγ, Sečen Qan Ayimaγ and Tüsiyetü Qan Ayimaγ, the demarcation process of the border has not been studied previously, and it is necessary to clarify how the border between this two ayimaγ was demarcated in order to fully comprehend the facts of the border demarcation policy in Outer Mongolia. Because of this situation, this paper examines the demarcation process of the border between Sečen Qan Ayimaγ and Tüsiyetü Qan Ayimaγ, using historical documents preserved in the National Central Archives of Mongolia. Specifically, I first overviewed the pastureland condition of Sečen Qan Ayimaγ at the time of the Sirege naγur Conference, and considered the reason that Sečen Qan Ayimaγ was not the subject of the border demarcation in the 46th year of the Qian Long reign. Secondly, I verified the demarcation process of the border between Sečen Qan Ayimaγ and Tüsiyetü Qan Ayimaγ. The following results were obtained. Although the Sirege naγur Conference was convened to settle the pastureland problem of ǰasaγtu Qan Ayimaγ, the conference incited the pastureland expansion of Sayin Noyan Ayimaγ as well as that of Sečen Qan Ayimaγ. The pastureland expansion of Sečen Qan Ayimaγ had a decisive effect on the pasture conflict between Sečen Qan Ayimaγ and Tüsiyetü Qan Ayimaγ. But the truth of the pastureland expansion and conflict in the eastern two ayimaγ was kept secret from the pastureland survey conducted by Bečingge in the 45th year of the Qian Long reign (1780). Accordingly, under the guise of not expanding pastureland after the fall of ǰegün γar, Sečen Qan Ayimaγ was excluded from the border demarcation of ayimaγ conducted by Batu in the 46th year of the Qian Long reign. In other words, the hiding of the truth about the pasture expansion and conflicts was a major factor why Sečen Qan Ayimaγ was excluded from the border demarcation of ayimaγ in the 46th year of the Qian Long reign. The pastureland conflict and border demarcation of the eastern two ayimaγ were reported to the Lifan Yuan 理藩院 in the 52th year of the Qian Long reign (1787) because these problems had not been successfully settled by the officials delegated by the leaders of the two leagues in the 46th and 47th years of the reign of Qian Long (1781-1782). Finally, the pasture conflict was settled by demarcating the border between the central southern area of the two ayimaγ in the 55th year of the Qian Long reign (1790) by Lawangdorǰi and Baǰung, who had been delegated by the Lifan Yuan in the 54th year of the reign of Qian Long (1789). In the demarcation process, the border area that had been demarcated at the Sirege naγur Conference, was redistributed and demarcated again in detail. As a result, the disputed area was returned to Tüsiyetü Qan Ayimaγ, and 77 pairs of Obuγ-a were set as the border between the banners located in the central southern area of the two ayimaγ except for the Central Banner of Tüsiyetü Qan Ayimaγ, Right-central Front Banner, Right-central Right Banner and Right-central Banner of Sečen Qan Ayimaγ. I have thus clarified that the four ayimaγ borders in Outer Mongolia were basically demarcated in the 55th year of the reign of Qian Long.