Let G be a graph of order n with vertices labeled as v1, v2, …, vn. Let di be the degree of the vertex vi, for i = 1, 2, …, n. The sigma index, which have been defined very recently, of G is (G) = ΣvivjE(G)(di  dj)2. In this paper, the sigma index of the Cartesian product, composition, join and disjunction of graphs are computed. We apply some of our results to compute the sigma index of some special graph classes.