  We give a simple proof for the rotational symmetry of ancient solutions of
Ricci flow on surfaces. As a consequence we obtain a simple proof of some
results of P.Daskalopoulos, R.Hamilton and N.Sesum on the a priori estimates
for the ancient solutions of Ricci flow on surfaces. We also give a simple
proof for the solution to be a Rosenau solution under some mild conditions on
the solutions of Ricci flow on surfaces.
