The fracture mechanics properties and dynamic crack propagation in polyoxymethylene plates are examined in this study. Experiments are carried out in terms of three-point-bending of beams with an initial central crack and also tensile testing of compact-tension test specimens. The test specimens were taken from plates that had been manufactured by injection-moulding. Test specimens were taken in different orientations in the test plates in order to investigate the possible influence of anisotropy. In both of the test specimens, dynamic crack propagation would initiate at a critical external load. In the three-point-bending tests, the crack would just propagate straight-ahead as a single crack, whereas in the compact-tension tests, the crack would branch into two main cracks after a short distance of single crack propagation. The experiments were also analysed using the standard finite element method as well as the phase-field method. The mechanical fields around the initial, stationary crack were analysed using a plasticity model and the standard finite element method, and dynamic crack propagation was modelled using a phase-field model. The simulations suggest that the surface energy in the material is considerably different for quasi-static crack propagation and dynamic crack propagation. The quasi-static surface energy was estimated to about 3kJ/m2, whereas the dynamic surface energy is expected to be 5-6 times lower than the quasi-static value. After the distinction between quasistatic and dynamic surface energies had been introduced, the phase-field simulations were able to reproduce the crack branching observed in the CT specimens.