The unstable shear crack propagation, which one encounters in the highly pressurized gas pipelines, was investigated experimentally and theoretically with the aid of DCB testing, which is composed of a double cantilever beam specimen and a spring. The dynamic aspects of propagating shear fracture were pursuied by measuring the change of load, the position of crack, the opening displacement at a loading point and the plastic strain distribution ahead of crack. Making use of experimental data, ductile crack opening displacement under high strain rate was calculated by the application of Dugdale-Muskhelishvili Model to DCB testing. The results are summarized as follows : (1) During the shear fracture propagation, a plastic strain distribution in front of crack depends on the material and does not depend on the crack velocity. An average strain rate ahead of a propagating crack is about 10²/sec. (2) An experimental estimation of the stress at crack tip under 10²/sec strain rate seems to give a stress corresponding to the conventional tensile strength. It is very plausible that the plastic instability and strain gradient ahead of a crack should be decisive factors in the mechanism of unstable ductile fracture. (3) Dynamic ductile δ_c is a material constant, and is insensitive to crack velocity. The δ_c, is well correlated to the uniform elongation in static tension test.