This paper presents a method for designing rectangular microchannel counter flow heat exchangers. The adopted methodology is based on two models: a one-dimensional model and a CFD model. A one-dimensional model for parallel plate micro heat exchangers is adopted to adjust the geometries of the rectangular microchannel counter flow heat exchanger. To this end a hydro-dynamically and thermally equivalent parallel plate heat exchanger is used. A more accurate CFD model serves to fit the parameters of the one-dimensional model. The related correction factors are presented for a range of pressure drops and axial conduction parameters. The present study provides quantitative data for the optimal plate dimensions and resulting maximal power density of a gas gas rectangular channel counter flow micro heat exchanger with effectiveness varied between 0.6 and 1. (C) 2014 Elsevier Ltd. All rights reserved.