Abstract: In situ synchrotron high-energy X-ray diffraction was employed to track the phase-fraction evolution and lattice-parameter variation of Fe₁₁Cr₈Ni₅Co₃Mo steel during laser-directed energy deposition. The results reveal that both quantities vary significantly with deposition layer and local thermal history. Interlayer analysis from the fifth to the 15th layer reveals austenite declines from 7.45 to 3.20 wt.% due to reduced heat accumulation, accompanied by progressive lattice contraction in both martensite and austenite from residual compressive stresses. This layer-dependent trend reflects the strong influence of local thermal history on phase stability during deposition. In the tenth layer, thermal cycling induces reversible phase changes: Martensite fluctuates (97.9 → 92.2 → 94.5 wt.%) while austenite inversely shifts (2.1 → 7.8 → 5.5 wt.%). Thermal gradients near the melt pool transiently expand lattice parameters, followed by cooling-induced contraction. C。。。orre。。。sponding nanoindentation measurements show layer-dependent hardness variations, with the fifth layer exhibiting the highest hardness of 5.72 GPa, attributed to the combined effects of thermal gradients and phase transformations. Overall, the phase stability, lattice evolution, and layer-dependent hardness are closely related to cyclic thermal history during laser-directed energy deposition.