The transient flow structure and heat transfer of plate-fin heat sinks cooled with dual piezoelectric fans (piezofans) have been investigated experimentally. The analysis integrated with the measurement of thermal resistance, velocity, and vorticity was conducted via high-speed particle image velocimetry (PIV) and infrared thermometry. The mechanism of the enhancement of heat transfer was deduced on varying the spacing, height, phase and orientation of the fan. As the ratio of fan spacing to the width of the heat sink (D*) and the ratio of the fan height to the height of the heat sink (H*) decreased, the thermal resistance decreased. The thermal resistance for the fans vibrating out of phase is invariably superior to the fans vibrating in phase. The thermal resistance in the parallel orientation was 22-27% less than in the vertical orientation because of the unobstructed ventilation of ambient air, dual fans and the heat sink. In contrast, the cooling performance in the vertical orientation decreased because the hot flow in the heat sink was isolated with only a small amount of entrained cold flow. The velocity induced by the dual fans decreased 30-50% because the buoyancy effect caused by natural convection annihilated the cold flow to move downward into the heat sink, resulting in a decreased cooling performance. The revealed mechanisms of interaction between the dual fans and the plate-fin heat sink shed light on the concept of optimal design for electronic cooling systems. (C) 2015 Elsevier Ltd. All rights reserved.