The effect of pre-treatment on carbide transformation and morphology of a medium-alloy high carbon steel (0.86C, 0.84Cr, 1.85W, 0.95Mo, 0.31V) with multi-elements was investigated in this paper. The results show that there are multiple types of carbides (M3C、M23C6、M7C3、M6C、MC) in the annealed steel. The morphology of carbide in the annealing microstructure largely relate to the process before annealing. Carbides with disperse spherical and short rod-like distribution exist in the ferritic matrix when the steels are annealed at 820−860°C. The spherical carbides in the ferritic matrix can be obtained by isothermal annealing at 680−720°C after heating at 800°C. The morphology of carbides with different pretreatment process relate to carbide transformation, which can be described by calculating based on phase equilibrium thermodynamic.