Heat transfer in the flow through a bundle of heated tubes are investigated numerically at Reynolds numbers around which the flow makes a transition from a steady state to an oscillatory one, as a model of micro heat exchanger. It is found that physical quantities such as the Nusselt number and the pressure loss exhibit discontinuous jumps with continuous change in the Reynolds number. The origin of the sudden changes in these physical quantities is identified to come from the existence of multiple stable solutions which leads to hystereses in these quantities. (C) 2010 Elsevier Ltd. All rights reserved.