Abstract: 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.