Gas turbine blade tips encounter large heat load as they are exposed to the high temperature gas. A common way to cool the blade and its tip is to design a serpentine passage with 180-deg turns under the blade tip-cap inside the turbine blade. Improved internal convective cooling is therefore required to increase the blade tip lifetime. In this paper, turbulent fluid flow and heat transfer through seven two-pass channels with and without guide ribs or vanes have been investigated numerically using Computational Fluid Dynamics technique. The influences of placing six kinds of guide ribs/vanes in the turn regions on the tip-wall heat transfer enhancement and the overall pressure loss of the channels are analyzed. The guide ribs have a height of 9% height of the channel while the guide vanes have a height identical to that of the channel. The inlet Reynolds numbers are ranging from 100,000 to 600,000. The detailed three-dimensional fluid flow and heat transfer over the tip-walls are presented. The overall performances of several two-pass channels are also evaluated and compared. It is found that the tip heat transfer coefficients of the channels with guide ribs/vanes are up to 65% higher than those of a channel without guide ribs/vanes, while the pressure loss might be reduced if the guide ribs/vanes are properly designed and located, otherwise the pressure loss is expected to be increased severely. It is suggested that the usage of proper guide ribs/vanes is a suitable way to augment the blade tip heat transfer and improve the flow structure, but is not the most effective way compared to the augmentation by surface modifications imposed on the tip-wall directly. (C) 2011 Elsevier Masson SAS. All rights reserved.