The authors have reported that minichannel flow system had high heat transfer coefficient, the reason of which was investigated experimentally and numerically for single and array minichannels combined with impingement flow system. The diameter D of the channel was 1.27 mm and length to diameter ratio L/D was 5. The minichannel array was so called shower head which was constructed by 19 minichannels located at the apex of equilateral triangle, the side length S of which was 4 mm. Single stage block was used to investigate the heat transfer without impinging flow system. Two stage blocks were used to compose an impingement heat exchanger system with an impingement distance of H. H/D ranged from 1.97 to 7.87. A comparison of heat transfer performance was made between minichannel flow system and impingement jet using the single and two stage heat transfer experimental data. It was found that heat transfer performance of the minichannel was equivalent to that of impingement jet. The mechanism of high heat transfer was studied numerically by the Reynolds averaged Navier-Stokes equation and k-ω turbulence model. The limiting stream-line pattern was correlated well to the surface heat flux distribution. The high heat transfer in the single minichannel was achieved by suppressing the development of boundary layer under strong pressure gradient near the channel inlet and by the formation of large recirculating flow system in the downstream plenum of the minichannel exit. These heat transfer mechanisms became dominant when the channel size fallen into the regime of minichannel. For the array of 19 minichannels, the high heat transfer around the channel inlet was also observed clearly in the target plate of the impingement jet where minichannels of second stage were bored to exhaust the fluid of impingement jet.