The knowledge of the flow behavior inside cooling channels is of great importance to improve design and performance of regeneratively cooled rocket engines. The modeling of the coolant flow is a challenging task because of its particular features, such as the high wall temperature gradient, the high Reynolds number and the three-dimensional geometry of the passages. In case of methane as coolant, a further complication is the transcritical operating condition of the fluid. In this thermodynamic regime large changes of the fluid properties can greatly influence the coolant flow-field and the heat transfer. Numerical simulations of transcritical methane flow-field in asymmetrically heated rectangular channel with high aspect ratio and strong wall temperature differences are carried out by a suitable CFD solver. Results are discussed in detail and compared with supercritical methane flow-fields. Finally the aspect ratio effect on methane flow is analyzed by comparison of four different rectangular cooling channel geometries with fixed hydraulic diameter and coolant flows with the same mass flow-rate per unit area. Emphasis is given to the comparison of fluid cooling performance and pressure loss. (C) 2011 Elsevier B.V. All rights reserved.