Flow characteristics in the entrance of plate-fin heat exchanger have been investigated by means of particle image velocimetry (PIV). The flow field was measured using the two-frame cross-correlation technique. Streamline and velocity contour graphs at different cross-sections were obtained in the experiment. The experimental results indicate that flow maldistribution in the conventional header is very serious, while the improved header configuration with punched baffle can effectively improve the uniformity. The flow maldistribution parameter in plate-fin heat exchanger has been reduced from 1.21 to 0.21, and the ratio of the maximum velocity to the minimum is reduced from 23.2 to 1.8 by installing the punched baffle. The results suggest room for the optimum design of plate-fm heat exchanger.