Kwon Gu's numerology, natural philosophy and world system are analyzed. Kwon was an important scholar who represented the Yongnam Namin scholar group in the early 18th century. He believed that it is required to study qi 氣 in order to understand li 理 which has no form. Since qi consisted of form and number, he paid much attention to numerology (象數 xiangshu). His numerology however was based on Neo－Confucian philosophy and did not aim to cultivate new one. Kwon's conservative attitude was also illustrated in his natural philosophy. Even though he offered some new interpretations to explain eclipse, meteorology, the ebb and the flow, and the shadow in the moon, he did so since there were no orthodox theories in these questions. His world system therefore was based on the orthodox Neo－Confucian interpretations with a few modifications.