To interpret motions by analyzing a series of illustrations is an interesting topic. This paper describes an approach for interpreting essential folding operations in illustrated books of origami. Since a folding operation such as ``folding up'' can be defined by a rotative surface, axis and direction, we proposed the algorithms to identify arrows and dashed lines which represent rotative surfaces, axes and directions in a typical book of origami. The arrow detection process consists of the arrow-head detection and arc detection. The former is accomplished by applying the distance transformation and border following to an input image, while the latter is composed of two phases: all paths which would correspond to arcs are found out in the thinned and approximated image first, an evaluative function is then applied to them to determine arcs. Dashed lines with different patterns are detected using a nearest-neighbor clustering method. A folding operation is finally interpreted based on the identified graphical primitives and the spatial relationships among them. The experimental results on several test images are presented. The effectiveness of our approach in generating meaningful interpretations of origami diagrams is clear from these results.