More often than not among Yeats``s works of the early 20th century In the Seven Woods (1904) or Responsibilities (1914) have been chosen as a watershed from his early to the middle period, and Green Helmet and Other Poems (1910, 1912) has been neglected or treated as a ``transitory`` work. Close investigation, however, reveals that Green Helmet is the ``new start`` with its new heroic tones and subject matters. And when we read the play that gives the book its name, ``The Green Helmet``, we find Yeats began to take Cuchulain as his ``mask``. He begins to try to identify himself with him. The causes may be many, but The Playboy of the Western World incident should have played a major role in transforming Yeats from an early Romantic poet to a ``mask-wearing`` middle period poet. Or ``the theatre business`` as a whole might have given him a new Weltanschaung. The heroic voiceof the poet in the book, What if I were not a poet,`` should be taken in conjunction with Cuchulain``s heroic action of the play.