In this study, the performance of a surface air-oil heat exchanger for an aero gas-turbine engine having plate- and pin-fin shaped geometries was investigated numerically. Basic heat-transfer and pressure-drop characteristics were examined using a simplified channel model. Performance of pin-shaped fins is compared with that of the plate fin as a baseline. Using a parametric study, optimal fin pitches of the pin-fin geometries in stream- and span-wise directions were determined. Finally, the high-speed bypass effect of the surface air-oil heat exchanger was calculated using the geometry of a real engine. Aero-thermal performance for such as the heat transfer rate, pressure-drop along the bypass stream of the engine, and distorted velocity boundary layer profiles, were evaluated quantitatively. The entropy generation rate due to the surface air-oil heat exchanger is summarized to assess the irreversibility loss inside the bypass stream and the cooler region. (C) 2015 Elsevier Ltd. All rights reserved.