Static and dynamic ductile crack propagation tests were carried out using thin single edge notched tension (SENT) specimens of carbon-manganese steel, each of which had a fatigue pre-crack or a sharp V-notch as a crack initiator. The crack tip opening angle (CTOA) was measured using digital images on the surface of the SENT specimens, and the critical values of CTOA for crack propagation decreased with increasing crack length while initial crack growth was still small. After the initial crack growth up to the distance of the specimen thickness, the critical CTOA remained almost constant. These tendencies were common in static and dynamic crack propagation specimens as well as fatigue pre-cracked and sharp V-notched specimens. There was no particular difference in the static crack propagation characteristics of both fatigue pre-cracked and sharp V-notched specimens. On average, it was observed that higher crack speeds affected the constant values of the critical CTOA by slightly reducing them. The constant CTOA tends to decrease with an increasing global constraint factor, and this suggests that the factor is insensitive to a crack starter, fatigue pre-crack or a sharp V-notch, but relatively sensitive to crack speed.