The Yijaenango, a lifetime journal of a Confucian scholar of eighteenth-century Korea named Hwang Yunseok, contains detailed records about Hwang’s study on mathematics and on his relationship with other yangban literati who had specialized interests in mathematics. Based mainly on this material, this article examines why and how a Confucian scholar in late eighteenth-century Korea studied mathematics, including Western mathematics introduced from China. Hwang’s interest in mathematics, as shown in his journal, was an extension of his interest in the traditional learning of “images and numbers (sangsu 象數),”an interest that he earned while studying Neo-Confucian texts compiled in the Complete Collection on Nature and Principle (Xinglidaquan 性理大全).He began his mathematical study from texts on Chinese traditional mathematics, notably, the Introduction to Mathematics(Sanxueqimeng 算學啓蒙), and as an extension of this study, he became interested in Western mathematical texts including the Elements of Geometry(Jiheyuanben 幾何原本), and the Essence of Numbers and Their Principles(Shulijingyun 數理精蘊) and the Thorough Investigation of Astronomical Phenomena(Lixiangkaocheng 曆象考成). This paper also examines Hwang’s records concerning the community of mathematicians in late eighteenth-century Seoul—the main figures in the field, their relations to one another, the circulation of books among them, and the trends of their mathematical studies.