The turbulent flow structure inside the entrance of plate-fin heat exchanger was characterized by CFD simulation and PIV experiment under the similar conditions. A series of velocity vectors and streamline graphs of different cross-sections are achieved for three distinct header configurations, involving conventional and improved configurations. The numerical and experimental results indicate that the performance of fluid maldistribution in conventional entrance is deteriorated, while the improved configuration with punched baffle can effectively improve the performance in both radial and axial direction. And the baffle on which the small holes are distributed in staggered arrangement is the first choice for the improvement. CFD results and PIV data arc in good agreement with each other. The results validate that PIV and CFD are well suitable to investigate complex flow patterns and the conclusion of this paper is of great significance in the optimum design of plate-fin heat exchanger. (c) 2005 Elsevier Ltd. All rights reserved.