The Szeged index of a graph is defined as , where is the number of vertices of lying closer to than to , is the number of vertices of lying closer to than to and the summation goes over all edges of . In this paper we find an exact expression for Szeged index of nanotubes, using a theorem of A. Dobrynin and I. Gutman on connected bipartite graphs (see [A. Dobrynin, I. Gutman, On a graph invariant related to the sum of all distances in a graph, Publ. Inst. Math. Nouvelle ser. tome 56 (70) (1994) 18–22]).