Abstract: Based on a plate diffusion welded rectangular micro-channel heat exchanger with convenient and efficient processing technology, a mathematic-physical model of the heat transfer unit of hot and cold straight channels was established, and the flow and heat transfer characteristics of supercritical carbon dioxide（S-CO₂） under different boundary conditions and different channel structures were investigated. Results show that with the increase of Reynolds number（Re）, the turbulences in the hot and cold straight channels enhance, the heat transfer performance improves, and the frictional resistance coefficient of the flow decreases. Compared with the flow and heat transfer characteristics in the rectangular channel with no grid, the turbulence effect and heat transfer characteristic are the best when the grid is horizontally arranged on both sides of the channels, and the comprehensive heat transfer enhancement factor（PEC） of the channel is relatively high. However, the PEC still needs to be further improved compared with that of the semi-circular channel.