In this article, we study the Schrödinger-Poisson system -ϵ2 Δu + V(x)u+ϕ(x)i = Q(x)u3, x ∊ R3-ϵΔϕ = u2, x ∊ R3, where &31013; > 0 is a parameter, V and Q are positive bounded functions. We establish the existence of ground states for ϵ small, and describe the concentration phenomena of ground states as ϵ → 0.