<p>FeNi-based MGs exhibit the most excellent room-temperature mechanical properties among different Fe-based metallic glasses (MGs) systems. However, the glass-forming ability (GFA) of FeNi-based MGs is much lower than that of their counterparts. Thus, uncovering the solidification and anomalous nano-crystallization behavior of FeNi-based MGs is crucial to the development of FeNi-based bulk metallic glasses with larger plasticity and critical size concurrently. Regarding this, a combination of complementary in situ synchrotron radiation high-energy X-ray diffraction, small-angle neutron scattering (SANS), and 3-dimentional atom probe tomography (3-D APT) is used to study solidification and nano-crystallization behavior of Fe₄₈Ni₃₀Mo₃B₁₉ liquid and MG. The time–temperature–transformation diagram was successfully depicted via melting spun, in situ solidification, and isothermal annealing methods. We found that the Fe₄₈Ni₃₀Mo₃B₁₉ MG can only be prepared via the melt-spinning method to obtain amorphous ribbons, which could contribute to the low activation energy for the nano-crystallization growth <i>E</i>_p. Moreover, during isothermal annealing, the anomalous slow growth behavior in kinetic of the γ-FeNi phase embedded in the amorphous matrix is caused by the Fe and Ni partitioning, and the Mo-enriched region around the nanosized γ-FeNi phase, which is revealed by 3-D APT. These results exhibit a new perspective for understanding the relationship between GFA and nano-crystallization behavior and provide feasible guidance for the development of new γ-FeNi-containing Fe-based BMG composites with desired mechanical properties and GFA.</p> Graphical abstract