The flow and heat transfer characteristics of fluid with two opposed flow directions in asymmetrical corrugated tubes (ACT) have been investigated through numerical simulations in this paper. The 3D ACT are modeled and studied numerically based on the k-epsilon model. A structured, non-uniform mesh system of hexahedral elements is created by the integrated computer engineering and manufacturing code (ICEM). The heat transfer working fluid in the tube side of the ACT is helium gas. In the numerical simulation results, the differentiations of flow and heat transfer performances between two opposed flow directions of fluid in the ACT are comparatively analyzed. The mechanisms behind the improvement of overall performances in the simulated ACT are discussed through investigating the details of velocity, temperature, pressure, turbulent kinetic energy and turbulent dissipation rate fields. Through comparing the performances and mechanisms between two opposed flow directions in the ACT, it is discovered that promoting of overall heat transfer performance is mainly determined by the large corrugation trough radii (rl) located at upstream side. The effect of various rl for the heat transfer and flow characteristic is also investigated to find the optimal one because the manufacturing difficulty is constrained by the increasing rl. Located rl/D = 0.6 in the upstream side of the tube side in the ACT is the more appropriate scheme. (C) 2014 Elsevier Ltd. All rights reserved.