In-situ micro-tension tests using high-speed imaging technology were employed to elucidate the localized shear deformation process after the onset of plastic instability in iron-based amorphous (AM) and ultrafine-grained (UFG) alloys. The AM specimen fractured at a tensile stress of ~3.20 GPa without plastic strain. The UFG specimen exhibited strain hardening after the occurrence of yielding at ~1.13 GPa. The tensile fracture stress and elongation-to-failure were ~1.65 GPa and 2.7%, respectively. The shear fracture planes of the AM and UFG specimens deviated from the maximum shear stress plane under uniaxial tensile stress. In-situ observation of the shear deformation process of AM specimen revealed that sliding off occurred almost throughout the entire specimen thickness. Therefore, the sliding-off process is helped by the applied normal stress, which suggests that it is caused by free-volume coalescence. By contrast, in the UFG specimen, yielding occurred with dislocation-based shear banding at an inclination angle of 45° with respect to the loading axis, which followed the Tresca criterion. Necking after shear band propagation formed a triaxial stress state, which resulted in a final shear fracture plane through void coalescence in the UFG specimens. Therefore, the deviation of the shear fracture plane from the maximum shear stress plane in the UFG specimen was determined by the development process of the strain under the triaxial stress state. A comparison of the post-plastic-instability behavior between the AM and UFG specimens suggests that the external control of triaxial stress conditions is key to betterment the formability of AM specimens.