In this paper we will show that for any map f on an infra-nilmanifold, the Nielsen number N (f ) of this map is either equal to |L(f )|, where L(f ) is the Lefschetz number of that map, or equal to the expression |L(f ) − L(f + )|, where f + is a lift of f to a 2-fold covering of that infra-nilmanifold. By exploiting the exact nature of this relationship for all powers of f , we prove that the Nielsen dynamical zeta function for a map on an infra-nilmanifold is always a rational function.