[en] J/ψ production has been measured in Au+Au and Cu+Cu at √(s_NN)=200 GeV by the PHENIX experiment at the relativistic heavy ion collider (RHIC) during 2004 and 2005, respectively, at mid-rapidity (|η|(le) 0.35) via J/ψ → e⁺e^- decay and at forward rapidity (1.2 (le)|η|(le) 2.2) via J/ψ → μ⁺μ^- decay. The nuclear modification factor (RAA) of J/ψ is presented as a function of the collision centrality for Au+Au collisions (final results) and Cu+Cu collisions (preliminary results) in both rapidity windows. These results are compared to SPS results at lower energy and to various theoretical calculations