This paper examined the process of establishing some oboos, which showed the pastureland’s border in Outer Mongolia during the Qing Period, and their role, using historical documents preserved in the National Central Archives of Mongolia. The following results were obtained. First, in Outer Mongolia during the Qing Period, specific procedures were necessary to establish the oboo, from the first step of preparing some stones and a piece of wood that is 2 m in height and 16 cm in width, to the final step of putting the wood in the stacked stones. Second, for the nomads, the oboos had two different aspects: if there were no negative effects on pastureland use, oboos were just stones and wood; however, if there were negative effects on pastureland use, nomads tried to expand their pastureland by moving or breaking the oboos, thus enforcing the nature of the oboo as a border. In other words, in Outer Mongolia during the Qing Period, the nomads recognized the border that was shown by oboos, and the oboo that appeared in the Mongolian nomadic society for the first time played a role in visualizing the pastureland’s border. The matter of moving or breaking oboos was then severely punished, based on Lifanyuan zeli『理藩院則例』or Da Qing lüli『大清律例』. All broken oboos were re-established, and it was revealed that oboos were closely watched by the government.