The transformation of primary MC and M2C carbides, during high-temperature treatment, was investigated by quasi in-situ observation of Cr4Mo4V steel. It was found that the V-rich rod-like MC carbides have great thermal stability, and disintegrating and coarsening are their dominant transformation mechanisms. The M2C carbides mainly possess rod-like (M2C-R) and lamellar (M2C-L) morphologies. The M2C-R carbides are Mo-enriched, and their transformation follows the paths: the separate precipitation of MC and M2C-R + γ→MC + M6C. The V- and Fe-rich MC carbides nucleates randomly in the interior of M2C-R before the nucleation of Mo- and Fe-rich M6C carbides, which is controlled by the diffusion of V. Soaking for longer durations can cause spheroidization and coarsening of the MC carbides, along with the dissolution of M6C carbides. The evolution of Fe- and Mo-rich M2C-L carbides consists of two stages: the partial dissolution of M2C-L into the matrix and the transformation of M2C-L→MC + γ. These novel findings contribute towards the understanding of how primary carbides evolve during heat treatment and provide a potential method for carbides refinement in Cr4Mo4V steel. Display omitted •The transformation behavior of primary carbides in Cr4Mo4V steel was investigated.•Disintegrating and coarsening are the dominant transformation mechanisms of V-rich rod-like MC carbides.•The M2C carbides mainly possess rod-like (M2C-R) and lamellar (M2C-L) morphologies.•The transformation process of M2C-R carbides follows the paths: the separate precipitation of MC and M2C-R + γ→MC + M6C.•The evolution of M2C-L carbides consists of the partial dissolution of M2C-L and the transformation of M2C-L→MC+γ.