By examining the knowledge of the celestial bodies and the universe, this paper tries to illustrate some aspects of Confucian understanding of Nature which prevailed during the middle era of Cho-sŏn朝鮮. I analyze the contents of Chang hyŏn-kwang's 張顯光(1554-1637) Y'ŏk-hak-do-sŏl which had no influence from the West and therefore preserved the more traditional view of the universe. I also examine his U-ju-sŏl in which he adopted Chinese cosmology and offered his own ideas. I show that Chang, in his two books, had a clear understanding of elementary principles of the celestial bodies and the calendar both of which had been included in the Confucian classics and commentaries, and that he owed largely to the Neo-Confucian cosmic principles from Hsing-li ta-ch'üan性理大全. Based on these, I argue that though Chang's understanding was restricted within Chinese Neo-Confucian ideas, he nevertheless had creative thoughts on the celestial bodies and the universe. In Chapter III, analyzing Y'ŏk-hak-do-sŏl, I show that he had ideas different from ChuHsi's(1130-1200) interpretation of the ancient "Ssu-yu 四遊" theory which, he thought, will repair the defects of Neo-Confucian theory. In Chapter IV, analyzing U-ju-sŏl, I show that he maintained that only the "Ku-gak 軀殼" could support the rationality of structure of the universe, and that he had the idea of the thickness of rotating "Won-ki 元氣"(rotating air) that could support the weight of the earth, and also that he clearly explained the infinity of the universe epistemologically.