Over 1.4 million foreigners live in South Korea now. The number of marriage-related immigrants is 125,673 and that of children of multicultural family is 107,689. These facts are brand new ones during the last five thousand year history of Korea. Such a change challenges Korean people and Christians to rethink respectively about their identity. This article aims at describing the current situation of immigrants, especially marriage-related immigrant women and multicultural family, identifying tasks for immigrant mission in Busan, Ulsan and Kyungsangnam-do and searching for the immigrant theology. Methodologies used in the article are: literature study, interview with specialists in immigrant mission, social biography of marriage-related immigrant women, missiology of encounter with other and Post-colonial Feminism. In Busan, the researcher chose four church/mission center/Christian NGOs participating in the immigrant mission in 2009. And he chose five mission centers/Christian NGOs participating in the immigrant mission in 2010. He interviewed with 31 members of multicultural families and with 9 specialists in the immigrant mission. Ninety-nine marriage-related immigrant women responded to the survey prepared by the researcher. The results of the social biography of multicultural family and the surveys of marriage-related immigrant women are: first of all, recovery of ‘normality’ of the multicultural family, for the family has been represented by the mass media as ‘abnormal’ one with family violence: secondly, Christian faith contributes for them to accommodate to Korean culture and to overcome conflicts between husband and wife of multicultural family: thirdly, the international marriage initiated by commercial agencies is blamed as money- exchange-marriage but the reality of such a family life is similar to that of a marriage arranged by a go-between: lastly, marriage-related immigrant women are faster learners of Korean language and culture than migrant workers. Tasks for immigrant mission are: to strengthen the multicultural family by overcoming cultural conflicts, to support for their children education, to establish multicultural faith community, to encourage marriage-related immigrant women to get a job, to suggest education programs for Korean Christians and Korean citizens, and to prevent from religious conflicts through peace education among religious groups.