<p>Velocity fields inside a three-hole fuel injector was analyzed by means of Proper Orthogonal Decomposition; POD method. The flow structure and hydrodynamic performance change under the development process of circular flow were investigated. The velocity field at center cross plane at sac area was visualized and measured by using a high-speed camera system and PIV analysis. The high-speed image detection enables us to evaluate sequential change of flow structure. The evaluation of flow structure was made by means of POD method, and the main feature of flow structure was able to be evaluated with small number of parameters. Two novel approaches for the application of POD method were proposed. One was a statistical method as demonstrated by left figure, and that enables us to examine all possible flow pattern appearance, and the other was a method that combined the evaluation based on hydrodynamic aspects and the geometrical feature evaluation as shown by right figure. Most of the existing POD applications has stayed to identify the main structure of field geometrically, but proposed methods succeeded to specify the target field status in connection with hydrodynamic aspects by demonstrating it on an evaluation space composed by the indexes obtained by POD analysis. Present study may demonstrate new perspectives of POD application at flow field analysis, and that may expand the possibilities of engineering utility.</p> Graphical abstract <p></p>