This study has investigated the rapid thermal oscillatory flow in an asymmetric micro pulsating heat exchanger (MPHE). In the design and fabrication of the MPHE, a transparent acrylic plate was attached to the stainless steel plate on which 14 parallel rectangular channels were formed. As working fluids, FC-72 and ethanol were tested. The fluid inside the micro channel moves rapidly with pulsating motions by swelling and shrinking of liquid slug/gas plug at evaporation and condensation sections. By measuring the temperature variations from start-up to steady state operation, the effective thermal conductivities were evaluated. Since the filling ratio of working fluid plays an important role in heat exchanger efficiency, the optimum filling ratio was found for each working fluid. From the flow visualization inside the MPHE filled with FC-72, two different flow modes oscillatory eruption mode and circulation mode occurs according to the filling ratio. The circulating motion which occurs at the optimum filling ratio, helps the heat transfer into the cooling water more active than the oscillatory eruption mode. (C) 2017 Elsevier Ltd. All rights reserved.