In this paper, we use the theory of simplicial groups to develop the Schur multiplier of a pair of groups (G, N ) to the Baer invariant of it, VM (G, N ), with respect to an arbitrary variety V. Moreover, we present among other things some behaviors of Baer invariants of a pair of groups with respect to the free product and the direct limit.Finally we prove that the nilpotent multiplier of a pair of groups does commute with the free product of finite groups of mutually coprime orders.