This paper examines Yeats's idea of hero and heroism in his Cuchulain plays. Cuchulain is the mythological champion of the ancient province of Ulster. He is the protagonist in a cycle of plays written by Yeats over a span of thirty-five years. Cuchulain became for Yeats a personal symbol for the heroic as well as the national ideal. He was not only his mask or alter ego but also the chief representative of that heroic age to which Yeats wished Ireland to aspire. Yeats significantly altered the Celtic legend to serve his dramatic purposes. He was concerned more with the nature of heroism than with the character and the life of the Ulster champion. So he was not interested in the hero's superhuman feats of arms or bravery which his source had emphasized. Instead he wished his countrymen to learn the hero's spiritual virtues: the unyielding spirit of challenge in At the Hawk's Well; selfless courage and sacrifice for his country in The Green Helmet; the comparison and contrast between the actual anti-heroic world and the heroic ideal in On Baile's Strand; true love and self-sacrifice of his wife in The Only Jealousy of Emer; forgiveness, mercy, unselfishness, and transcendence of the fear of death in The Death of Cuchulain. Cuchulain's heroism consists in a combination of daring, gaiety, strength and beauty, and he is a free man, a challenger who, whether he wins or loses a specific battle, is ultimately victorious over himself and over others. The hero is freed from every form of hesitation, both moral and physical. The essence of Yeats's heroism is sacrifice and the creative joy separated from fear.