This article reviews the competition of two natural family planning methods in the mid-1970s when the Catholic Natural Family Planning program was underway in Korea. The Catholic Church, emphasizing the natural law, has recommended Natural Family Planning (NFP), a method of regulating childbirth by abstinence during the fertile period, since the mid-twentieth century. However, a group of gynecologists working at St. Mary's Hospital, a Catholic general hospital in Korea, questioned the utility of NFP. As an alternative, they proposed the method of Ovulation Regulation (OR), which regulates the menstrual cycle by inducing ovulation with steroids agents. This seemed to be no different than contraception with oral contraceptives disapproved of by the Catholic Church, but many doctors who advocated OR thought that this could be a new 'natural' family planning method to replace NFP. What is noteworthy here is the fact that not only NFP advocates, but also OR advocates attempted to justify their methods based on the authority of the 'nature.' In the debate over natural family planning methods, nature's legitimacy was given premise, not the object of doubt. Rather, the issue was the definition of nature. First, 'nature' in NFP signifies 'innate nature,' which excludes human intervention. According to this point of view, OR with steroids agents could not be natural. On the contrary, a group of doctors who advocated OR considered nature 'primal completeness.' If the natural order of the menstrual cycle could be restored, the artificial intervention of the administration of steroids was not a problem. Thus, both groups defended their arguments by redefining nature, rather than raising an issue of nature itself. The competition between 'innate nature' and 'complete nature,' a proxy war between NFP and OR, resulted in the victory of the former as the meaning of nature became fixed. Advocates of NFP pointed out that OR inhibits other physiological functions in the process of inducing ovulation, suggesting that the idea of 'complete nature' could never be achieved. The meaning of nature could no longer be controversial. Since the intervention was unnatural, nature meant innateness, the absence of intervention. Accordingly, the Catholic Bishops of Korea approved the Billings Method, a kind of the NFP, as the official family planning method, and gynecologists at St. Mary's Hospital of Korea also focused on the development and supplementation of the Billings Method. In short, the debate over the methods of natural family planning in mid1970s Korea was a clash of 'innate nature' and 'complete nature.' As a result, this confirmed the limitations of medical practice and reconfirmed the power of magisterium, the church's authority over medical practice.