The internal cooling systems usually consist of multiple types of structures such as ribs, pin-fins and impinging jets. The influence from upstream features could cause a significant change in performance for individual cooling technologies. The present study numerically investigated the entrance effect of rib induced vortices on pin-fin arrays' heat transfer performance. Four different rib configurations were employed in the upstream of pin-fin arrays. The flow field and heat transfer of each case were simulated by using the commercial software ANSYS CFX with the SST k-omega turbulence model. The investigation revealed that the ribs induced secondary flows significantly influenced the heat transfer distribution and flow field in pin-fin arrays, especially caused strong heat transfer variation along the spanwise direction. The presence of ribs came with enhancing heat transfer and reducing the pressure loss of pin-fin arrays. The 90 degrees rib had the best overall performance among the simulated configurations, outperforming the baseline by approximately 51%-77%. These results indicated great potential to improve pin-fin heat transfer by optimizing upstream features, and could benefit future design for the industries. (C) 2018 Elsevier Ltd. All rights reserved.