Laser-assisted metal processing has received great attention for repair, manufacturing, and surface modification in various industries. However, the mesoscopic segregation and oxidation phenomena during these processes require further investigations. In present work, the H13 tool steel is remelted by a laser beam. The generation of oxide layer, the profile and the mesoscopic segregation of the molten pool, Marangoni convection, the and hardening effect are studied by an experimentation combined with a multiphysics coupled simulation. An oxide layer was formed on the remelted surface with a thickness of ∼ 60 µm in the middle and ∼10 µm in the edges. The lightweight elements (Si, V, Cr, and Mn) float upon the molten pool driven by both buoyancy and Marangoni convection. In addition, due to their higher redox activity, they tend to have a higher proportion in the oxide layer with respect to the base metal. As a consequence, elemental mesoscopic segregation appeared in the remelted zone. The oxide layer shows an average hardness of 11.8 GPa, which is similar to that of the remelted zone (11.7 GPa). However, the brittleness of the oxide layer leads to cracks, hence deteriorating its mechanical properties. Together, these results bring a further fundamental understanding of the mesoscopic segregation that is attributed to the Marangoni convection and oxidation phenomenon during the laser processing of alloys, which might be a critical factor to optimize the mechanical properties. Display omitted