Generally many Christians think conversion as a religious experience related to salvation and a mysterious event which happens only once in person’s life. Christians believe without any doubt that conversion is possible only by the work of God. However, they also agree that there is psychological phenomena in the process of conversion. Therefore, the purpose of this article is to approach Christian conversion spiritually and psychologically to get comprehensive understanding on conversion for modern Christians. In chapter 1, conversion is explained from the viewpoint of traditional faith. Here the terms, new birth, regeneration, renewal, turning, are introduced. Even though there are some different expressions on conversion, there is one common ground, that is, “recovery of relationship with God.”Here some cases are suggested to show that traditional understanding on conversion has limitation to explain fully about religious experience called conversion. In chapter 2, the reason why we need to approach conversion psychologically is suggested. In chapter 3, conversion is analyzed psychologically through William James, Carl Jung, Lewis Rambo, and some other psychologists. James saw conversion as a unification of a divided self and focused on the radical conversion rather than gradual one. Even though it seems to happen suddenly, however, there can be several stages in the process of conversion. Each stages of conversion is explained here. In chapter 4, the limitations and contributions of psychological approach to conversion is analyzed. Especially the importance of gradual conversion, the meaning of conflict in conversion, danger of negative conversion is treated here. The reason to clarify limitations and contributions is to help modern Christians, who are influenced deeply by psychology, have a deep and healthy understanding on conversion which will lead them to better Christian life.