In this paper, the writer investigated the thoughts on military art and science with a focus on the typical books on military art and science, which was published in the latter period of Joseon, and the discussion of literati in that time. Joseon had been happy to enjoy the piping times of peace for about 200 years ever since the establishment of the dynasty. However, having had to gone through two major wars, the Joseon Dynasty, revolving around scholarly people, had awakened the limits of military art and science of Joseon. It can be said that the countermeasure against Japanese pirates, which were reflected in the "Jingbirok" (懲毖錄 - Records of the 1592 Japanese Invasion) written by Yu Seong-ryong, and the experiences of war had formed the basis of the thoughts on military art and science in the latter period. Regrettably, there were no suggestions or proposals of preparing countermeasure against Japanese raiders in the books of military art and science in the early period of the Joseon Dynasty. Meanwhile, as the argument about the battle formation in the early period of Joseon, the process of establishing the military science had not gone smoothly in the latter period of Joseon. Right after the Japanese invasion of 1592, "Gihyo-Sinseo" (紀效新書 - New Text of Practical Tactics written by Cheok Gye-gwang) was brought into the country by the army of Ming (明) Dynasty. At first, this was used in the form of its original edition, or of abstract version in the military drill. But, later, it was published under the title of "Byeonghak-jinam" (兵學指南 - Military Training Manual about Action Rules by combat situation). This book, same as in Zhejian (浙江) province in China, had achieved a positive effect on counteracting the Japanese raiders in our country. However, these military tactics were conflicted with "Owi Jinbeop" - Rules of Deployment of the Five Military Commands, which had been handed down ever since the early period of the Joseon Dynasty, and, at the same time, it was pointed out that those tactics would not be able to apply to the situation uniformly, since Korea and China were geographically different. Furthermore, having gone through Manchu Invasion of 1636 (丙子胡亂, Byeongja horan) Joseon had used "Yeonbyeongsilgi" (練兵實記 - the Actual Records of Training Army), which was compiled in China on the basis of the experiences of wars against the nomad, including Mongolia and so on. And, this had become a typical training manual together with "Byeonghak-jinam". King Yeong Jo and King Jeong Jo of the Joseon Dynasty had tried to establish uniformity in military training by publishing the books of military science representing the latter period of Joseon such as "Sokbyeongjangdoseol" (續兵將圖說- Revision of the Illustrated Manual of Military Training and Tactics,) "Byeonghaktong" (兵學通 Book on Military Art and Science,) "Byeonghakjinamyeonui" (兵學指南演義 - Commentary on 'Byeonghak-jinam') and "Muyedobotongji"(武藝圖譜通志 - Comprehensive Illustrated Manual of Martial Arts,) and so on. King Jeong Jo had actively participated in the arguments in those days. So then the arguments that had been continued for about 200 years, ever since King Seon Jo, put to an end. To sum up the distinctive features of military art and science in both former and latter period of the Joseon Dynasty, in the former period of Joseon, the reasoning military science was proceeded with the initiative of civic official based on "Mugyeongchilseo"(武經七書- the Seven Military Classics). However, in the latter period of Joseon, "Gihyo-Sinseo"(紀效新書 - New Text of Practical Tactics written by Cheok Gye-gwang) had served as a momentum, and also comparatively a large numbers of military official had participated in arguments, so then such an occasion had made the military science turn into the Practical Theory. Meanwhile, King Sejo and King Jeong Jo had played a leading role in the process of establishing the theory of military science of Joseon, however, there are something in common that their succession to the throne was not smooth. This is the part that reminds us "War is an extension of politics," the thesis of Clausewitz