An　investigation　on　heat　transfer　of　a　micro-jet　array　impinging　smooth　and　pin　fin　surfaces　was　conducted.　The　experimental　results　indicate　that:　(1)　the　dimensionless　nozzle-to-target　spacing　(H/d)　has　notable　effects　on　heat　transfer　coefficient　of　both　free-surface　and　submerged　jet　impingement,　and　heat　transfer　may　be　enhanced　with　increasing　Reynolds　number　or　decreasing　dimensionless　jet-to-jet　spacing　(S/d).　(2)　the　enhancement　factor　(Ε)　is　not　influenced　by　H/d　for　free　surface　jet,　but　it　is　sensitive　to　variation　of　H/d　for　submerged　jet.　For　both　free-surface　and　submerged　jet　impingement,　Ε　increases　with　increasing　Re.　(3)　The　convection　resistance　decreases,　at　first　rapidly　and　then　slowly,　with　increasing　liquid　flow　rate.