  Margalit and Schleimer constructed nontrivial roots of the Dehn twist about a
nonseparating curve. We prove that the conjugacy classes of roots of the Dehn
twist about a nonseparating curve correspond to the conjugacy classes of
periodic maps with certain conditions. Futhermore, we give data set which
determine the conjugacy class of a root. As a consequence, we can find the
minimum degree and the maximum degree, and show that the degree must be odd.
Also, we give Dehn twist expression of the root of degree 3.
