We study structural,mechanical,and electronic properties of C20,Si20 and their alloys (C16Si4,C12Si8,C8Si12,and C4Si16) in C2/m structure by using density functional theory (DFT) based on first-principles calculations.The obtained elastic constants and the phonon spectra reveal mechanical and dynamic stability.The calculated formation enthalpy shows that the C-Si alloys might exist at a specified high temperature scale.The ratio of B/G and Poisson’s ratio indicate that these C-Si alloys in C2/m-20 structure are all brittle.The elastic anisotropic properties derived by bulk modulus and shear modulus show slight anisotropy.In addition,the band structures and density of states are also depicted,which reveal that C20,C16Si4,and Si20 are indirect band gap semiconductors,while C8Si12 and C4Si16 are semi-metallic alloys.Notably,a direct band gap semiconductor (C12Si8) is obtained by doping two indirect band gap semiconductors (C20 and Si20).