This study analyzes the processes and the needs of public library services for multicultural populations based on cooperation between public libraries and local organizations. In-depth interviews with public librarians and officers of local organizations were conducted for this case study. Result shows that cooperation was inevitable because there was not enough library personnel compared to the amount of services they have to provide, and the librarians did not have proper skills to serve the diverse populations. Public libraries were cooperating with many local organizations including the Multicultural Family Support Center. Gatekeepers of ethnic communities were playing important roles in promoting and inviting participants for library programs. Many children and young adults with immigration background gained positive views on reading and libraries after participating library programs based on cooperation between public libraries and local organizations, which was recognized as the best achievement of cooperation by many interviewees. Despite many good results of cooperation, how to get to the majority of diverse populations who are not library users yet is still a question mark. Interviewees for this study all agreed that current library service has limitations and insisted a whole new method to invite immigrants and their families to public libraries should be created.