The structure of reinforcing particles of niobium carbide in a cast superalloy based on the Fe-Cr-Ni-C system, modified with Nb and Ti, was studied by optical and electron microscopy. Niobium carbide particles in the cast alloy structure are mainly multiphase polycrystalline clusters with heterogeneous chemical composition and crystal structure. The disorientation angle between the individual crystals forming carbide particles is 30–60°. The polycrystalline nature of carbides is probably associated with significant thermal stresses occurring at the interfaces in the alloy structure during the initial cooling of the ingot. For an explanation of the polymorphism of niobium carbide clusters, further analysis on the basis of structural and geometric crystallography is required.