This thesis examined the concept of assimilation policy and the implementation period thereof. Furthermore, this thesis reviewed the period classification and the characteristics of each period in relation to the assimilation policy. There is some dissent on the implementation period of assimilation policy during the period of Japanese occupation. This disagreement is mainly derived from that many scholars have a different concept on the assimilation policy. To review the concept of assimilation policy, this thesis compared and examined the ruling mechanism of Japan with the British and French colonial system. This thesis then examined the characteristics of the colonial ruling policy of the Japanese Empire. Japanese Empire’s ruling system for Joseon was similar to the crown colony of the United Kingdom. However, its contents were founded on the basis of the French assimilation policy. This is the mixed type of colonialism based on the British colonial system and the French colonial system. That is to say, Japanese Empire had a very unique ruling mechanism. The Japanese Empire annexed the Korean peninsula permanently even from the early stage of Japanese colonial era. Moreover, the Japanese Empire intended to assimilate Korean people. The Japanese Empire believed that it would never be able to rule Joseon permanently unless it assimilated Korean people. Therefore, it proceeded with the step-by-step process under its ambitious assimilation plan even from the early stage of Japanese colonial era. On this account, the concept of assimilation policy should be established in consideration of the colonial ruling mechanism of Japanese Empire that intended to rule Joseon permanently. An assimilation policy can be deemed to have begun when a colonial policy to build a basis of civil assimilation and national assimilation is implemented with assimilation ideology without recognizing the tradition, uniqueness and identity of a colony. In this regard, this thesis deemed that an assimilation policy was implemented if a phased assimilation policy was implemented rather than simply expressing an intention of assimilation. According to the aforementioned concept of an assimilation policy, the Japanese Empire’s assimilation policy had been implemented all the time during the Japanese colonial era. This thesis classified the period of Japanese occupation into the three phases on the basis of its assimilation policy. The first phase was from the Japanese annexation of Korea to the March 1st Movement. This was “the period of building a basis of assimilation by forceful ruling”. The second phase was from the March 1st Movement to the outbreak of Sino-Japanese War. This phase was “the assimilation policy phase dominated by conciliatory gestures”. During the second phase, the Japanese Empire strengthened the Japanese language education in schools. That is, the level of national assimilation was enhanced compared to the first phase. The third phase was from the outbreak of Sino-Japanese War to Korea’s liberation. This phase was “the blatant assimilation policy phase”. During the third period, the Japanese Empire had forced Korean people to accept the colonial policy of subjecting to Japan. Thus, the Japanese Empire had blatantly and forcefully tried to assimilate Korean people into the Japanese culture.