In this paper, the crack propagation characteristics in AISI 4340 steel with different strength and toughness have been investigated in quasi-situ via 3D X-ray tomography during fracture toughness testing. Three kinds of crack propagation characteristics were observed by the 3D image analysis, which were determined by the competition between cleavage fracture and voids growth/coalescence. It is found that the main crack would connect the micro-voids near the crack tip by cleavage fracture if the local stress achieves the cleavage fracture strength, resulting in rapid crack propagation. However, the growth and coalescence of micro-voids ahead of the crack tip would be linked with the main crack by strain-controlled, leading to the ductile fracture. The above two mechanisms would exist simultaneously in materials, which might be explained by the competition balance between strain and stress. Studying on transition of crack propagation mechanisms in materials would further understand the essence of fracture toughness. It will provide new insight into the fracture behavior of engineering materials and some new materials such as nanostructured materials and metallic glasses. Additionally, based on the energy principle, we propose a quantitative relationship between the microscope fracture surface morphology and fracture toughness of materials.