Jet impingement cooling with carbon dioxide at supercritical pressures on micro pin fin structured surfaces was studied experimentally to enhance the impingement cooling heat transfer. The effects of the microstructure, mass flow rate and heat flux on the heat transfer were studied using the MEMS (Micro-Electro-Mechanical System) based silicon chips for heating and simultaneous surface temperature measurement. The pressure was 7.85 Mpa, which is higher than the critical pressure of carbon dioxide of 7.38 MPa. The jet inlet temperature was 19.0 degrees C, which is lower than the pseudocritical temperature of 33.78 degrees C at 7.85 MPa. The mass flow rates were 14.4 kg/h and 20.7 kg/h with the heat flux varying from 0.01 to 0.35 MW/m(2). The results showed that the surface area increased by the micro structured surfaces was the major reason for heat transfer enhancement with the circular pin fins giving greater heat transfer enhancement than the square pin fins with the same total surface area. The high specific heat fluid layer near the cooled surface at supercritical pressures enhanced the heat transfer rate with the average heat transfer coefficient first increasing with increasing heat flux and then decreasing with further increases in the heat flux.