The purpose of this thesis is to study the double self in Eugene O'Neill's mask plays: The Great God Brown, where one man plays two roles and Days Without End, where two men play one role. Eugene O'Neill wanted to express visually such confusion, conflict, fusion, and displacement of double self as the psychological struggle between inner self and outer self by changing the mask figure and putting on another mask. The ultimate method overcoming the conflict and struggle of double self is to achieve the freedom and the salvation from the mask, and O'Neill suggests the way of them through Cybel, Earth Mother in The Great God Brown for Dion and Brown, and Elsa, the Virgin Mary in Days Without End for John. (Wonkwang University)