CO_2 as a natural refrigerant is proposed for one of alternatives and recommended in car-conditioners and heat pumps for hot-water supply system. However, unsolved problems are remained to develop the optimum designing method of heat exchanger. In this study, in order to reveal the pressure drop and boiling heat transfer of CO_2 flowing through the micro channel, the test channel was used stainless steel tube with 400mm and 800mm in length and the inner diameter of 1mm and horizontally placed. We investigated the flow and the heat transfer characteristics in the micro channel and compared experimental results with the correlation formulas for the conventional scale channels.