© 2018 Elsevier Ltd Laser 3D printing is a promising technique to repair damaged Ni-based superalloy components. However, the occurrence of heat affected zone (HAZ) cracking severely limits its applicability. Here we unravel the cracking mechanism by studying the element, phase, defect, and strain distribution around an intergranular crack that initiated from the primary HAZ. Using synchrotron X-ray Laue microdiffraction, we measured high tensile strain/stress transverse to the building direction in both the primary HAZ and the cladding layers, as well as high-density dislocations, which resulted from the thermal contraction and rapid precipitation of γ′ phase. The crack initiated because the transverse tensile strain/stress tore up the liquid film formed by the low-melting point preexisting phases in the primary HAZ, such as γ/γ′ eutectics and coarse γ′ precipitates. The incoherent carbide particles were frequently observed near the crack root as local strain concentrators. In the cladding layers, micro-segregation could not be completely avoided, thus the hot crack continued to propagate over several layers with the assistance of the transverse tensile stress. Our investigations provide a useful guideline for the optimization of the 3D printing process to repair Ni-based superalloys with high susceptibility to hot cracking.