Let Kn,n denote the complete bipartite graph with n vertices in each partite set and Kn,n +I denote Kn,n with a 1-factor added. It is proved in this paper that there exists an m-cycle system of Kn,n + I if and only if n 1 (mod 2), m 0 (mod 2), 4 m 2n and n(n + 1) 0 (mod m).