The purpose of this dissertation is to research and evaluate Oliver R Avison’s mission works with ecumenism angle. There has been only a few research about him. And this dissertation covers 4 themes of his works. First, this dissertation deals with the identity of Oliver R. Avison. Most of all, Avison was a Christian missionary. Therefore, the works concerned with Avison should be totally researched by the mission itself. Avison did his utmost efforts to preach the gospel. For preaching the gospel, and training the Korean medical doctors, Avison devoted himself to Jejoongwon, Severance Hospital, and Severance Union Medical College. And after the death of Horace Grant Underwood, Avison had tried to unify Severance Union Medical College, Chosen Christian College and other Christian institutes for the effective mission works. Avison surpassed others both in character and ability during his mis-sion period. He was a brilliant Hospital Director, a great educator, a good manager, a outstanding political leader, and a powerful sponsor of the Korean patriots. Second, this dissertation deals with the core thoughts and values of Avison. That was ‘Union and Ecumenism’. Third, this dissertation deals with Avison’s detail works based on his core thoughts and values. For the powerful mission, Avison had tried to unify with even non christians and non christian institutes such as the Chosun government and the Japanese Government General of Korea. Avison had also tried to unify with christians and christian institutes across all the denominations and to do union works, based on his ecumenism. The mission works by Avison was not one sided supports, but to preach gospel through training the Korean medical doctors. Forth, this dissertation deals with meaning of the Avison’s works in the past and now. Avison devoted himself to the mission for the both expanding Christian religion and happy life in Korea. For this, Avison had tried to do union works across all the denominations, and done his best efforts to unify Korean churches. And he suggested innovative concept of union based on ecumenism to the world churchy society. Avison made union hospital and college, tried to invite outstanding professors and doctors to the union institutes such as Severance Hospital, Severance Union Medical College, and Chosen Christian College. And Avison trained many christian professors and students who could succeed his concept of union. Today, Korea was separated with north and south, east and west, the poor and the rich, the conservatives and the progressives, not only in the normal society but also in the churches. To solve these problems, it would be perfect solution that Avison had suggested. It is union and ecumenism.