<p>One of the most impressive advances in the field of carbon clusters in recent years is the successful synthesis and characterization of a ring consisting of carbon atoms, named cyclo[n]carbon, on the surface of a crystal. Doping with other atoms can greatly affect the structure and electronic properties of host clusters, and can form endohedral fullerenes, doped graphene, heterogeneous carbon ring, etc. Whether the structures of Be and Mg atom doped carbon clusters are ring-, fullerene- or graphene-like configurations is interesting. In this study, the structures and properties of M₂C_n (M = Be, Mg; n = 8–20) clusters are determined by combining structural search and density functional theory. Structural transition from single- or double-ring structures to planar honeycomb-like structures occurred in M₂C₁₅. Be₂C_n with even numbers for n values have relatively high vertical ionization energy and low electron affinity, and the binding energy of M atoms decrease rapidly when the cluster size n is larger the turning point of structural transition. The highest occupied molecular orbital − lowest unoccupied molecular orbital (HOMO–LUMO) gap of M₂C_n decreases with n value. Moreover, bond length, Mayer bond order, charges on M atoms, and aromaticity are studied. Be₂C₁₅ and Mg₂C₁₅ show remarkable aromatic character, and differences in aromaticity between Be₂C₁₆ and Mg₂C₁₆ are comprehensively analyzed.</p>