M50 steel is primarily used for manufacturing the main shaft bearings of aero engines. However, the fatigue property of M50 steel affects the service life of shaft bearings owing to their operation in the environment with high temperature, high rotation speed, and high contact stress. Inclusions and large-sized carbides are proved to be the primary reasons that cause fatigue cracking. Nevertheless, inclusions in M50 steel and fatigue failure due to inclusions are substantially reduced with the rapid development of metallurgical technology and metallurgical equipment in recent years. M50 steel contains high fractions of Cr, Mo, and V elements, which are easily enriched and can form primary carbides. The primary carbides in M50 steel are hard and brittle and cause stress concentration under external load, thereby accelerating the initiation and propagation of fatigue cracks. Currently, the large-sized primary carbides in M50 steel play an important role in reducing the service life of bearings and have attracted substantially research attention. The present investigation focuses on the decomposition mechanism of large-sized M₂C primary carbide in M50 steel to reveal the carbide-refinement mechanism during high-temperature heat treatment. In addition, M₂C primary carbide in M50 steel was systematically characterized by SEM, EPMA, and TEM, and its decomposition mechanism at 1160-1250^oC was studied. The difference in chemical composition of different M₂C primary carbides and its effect on the decomposition mechanism were also explored. Three forms of M₂C carbides in M50 steel were revealed: the rod-like carbide, the lamellar-like carbide, and the block-like carbide. In these three M₂C carbides, the content of Fe increased, while the content of Mo decreased successively. The difference in chemical composition and morphology of these three M₂C carbides led to the different microstructure-evolution process when heat-treated at elevated temperature. When the steel was heat-treated at 1160-1180^oC, only the M₂C carbides with high Fe content decomposed and a few of the carbides transformed to MC carbide. The growth rate of MC carbide was extremely low at this temperature. When the steel was heat-treated at 1210^oC, most of the M₂C carbides decomposed after 20 h. The growth rate of MC carbide also increased rapidly, and a large amount of large-sized MC carbides were found. Further heat treatment of steel at 1250^oC resulted in the decomposition of all M₂C carbides and the absence of large-sized primary carbides in the microstructure. However, a large amount of newly born M₂C carbide, formed due to the melting of the matrix and re-solidification, were found in the microstructure.