Dynamic crack propagation in a brittle polymer, poly(methyl-methacrylate) (PMMA), was studied using the method of caustics in combination with a Cranz-Schardin high-speed camera. Four different types of specimen geometry and loading method were employed to achieve the crack acceleration, deceleration, and/or reacceleration processes in one fracture event. The dynamic stress intensity factor K (ID) and crack velocity were obtained in the course of the crack propagation and the corresponding relationship was determined. The effect of the crack acceleration and deceleration on the K (ID)-velocity relationships was as follows: (1) the variations of K (ID) and the velocity were strongly influenced by the specimen geometry and loading method; (2) the velocity change was qualitatively in accord with K (ID); (3) K (ID) for a constant crack velocity was larger when the crack decelerated than it was when the crack accelerated or reaccelerated; (4) K (ID) for an acceleration-free crack was uniquely related to the velocity; and (5) K (ID) could be expressed as two parametric functions of the velocity and acceleration.