Hot cracking is a severe defect that occurs during the repair or manufacturing of directionally solidified superalloy by laser metal deposition. Understanding the cracking behavior and mechanism is vital to avoid these defects. In this study, a directionally solidified superalloy, IC10, was investigated by laser re-melting. Hot cracking not only occurred at high angle grain boundaries, but also the low angle grain boundaries. A critical angle, below which the hot cracking may not occur, exists within the scope of low angle grain boundaries. Hot cracking can be ascribed to the coupled effects of a liquid film, stress concentration, and particles enriched with Cr and Mo. The formation of liquid film can be ascribed to the liquation of low melting point structures. The stress concentration around the grain boundaries provides the driving force for crack initiation and propagation. The liquid feeding can be inhibited by particles enriched with Cr and Mo, which promote the initiation of cracks. These findings provide technical support for achieving successful repair and additive manufacturing strategy of the non-weldable directionally solidified superalloys. •Hot cracking is the coupled effects of a liquid film, stress concentration and particles enriched with Cr and Mo.•Misorientation angle of dendrites affects the grain boundary energy, which decides the stability of liquid film.•The strain localization factor at liquid film of substrate is larger than that of re-melted zone.•Particles enriched with Cr and Mo can promote the hot cracking by inhibiting the liquid feeding.•The appropriate laser power and scanning speed are about 400 W and 8 mm/s, respectively.