The method of caustics (shadow spot method) has proven to be a powerful optical method to measure stress intensity factors in static and dynamic fracture-mechanics problems. In this paper, high-speed photographs of the caustic patterns for dynamically propagating cracks were taken by a laser caustic method which can be rapidly synchronized to the initiation of the brittle fracture event. The dynamic stress intensity factors were evaluated on the basis of a theory of caustics for mixed-mode fast-running cracks, which was developed by the present authors. Influences of the length of the double cantilever beam (DCB) specimen were thoroughly investigated. It was found that the crack arrest toughness values, KIA, in shorter specimens were larger than those in longer specimens.