Let S be a closed Riemann
surface of genus g(≥ 2). It is known that the maximum value of the orders of
automorphisms of S is 4g + 2. In this paper we determine the orders of
automorphisms of S which are greater than or equal to 3g, and characterize
those Riemann surfaces having the corresponding automorphisms. Except for
several cases, such Riemann surfaces are determined uniquely up to conformal
equivalence.