C-2 hydrocarbon separation is one of the most important industrial issues in the petrochemical industry. As an energy-efficient method, the C-2 hydrocarbon separation using metal-organic frameworks (MOFs) has attracted considerable attention. In this work, adsorption and separation performances of C2H2/C2H4 and C2H2/C2H6 mixtures in four MOFs, including Zn-2(bdc)(2)(bpy), Zn-2(bdc)(2)(bpy)-ZnF2, Zn-2(tmbdc)(2)(bpy), and Zn-2(tmbdc)(2)(bpy)-ZnF2, were investigated by grand canonical Monte Carlo methods. The effects of pore sizes and surface chemical functions of MOFs on C-2 hydrocarbon separations at room temperature were deeply analyzed. Besides, density functional theory calculations were carried out to clarify the underlying mechanisms in adsorption and separation processes. The results indicate that our newly developed Zn-2(tmbdc)(2)(bpy)-ZnF2 shows ultrahigh C2H2/C2H4 and C2H2/C2H6 selectivities. Particularly, the C2H2/C2H(4) selectivity of Zn-2(tmbdc)(2)(bpy)-ZnF2 is nearly a hundred times higher than that of NOTT-300. This can be mainly attributed to the combination of the facilitations of chelated ZnF2 groups to C2H2 adsorption and the steric hindrance effects of -CH3 groups to C2H4 molecules. The mechanism analysis shows that the C-2 hydrocarbon prefers the site on the top of the ZnF2@bpy linkers owing to the strong interactions from ZnF2 groups. The strategies of surface function adjustment and pore-size control in this work can be applied in the modifications of other existing MOFs, as well as the design and synthesis of novel MOF materials for ultrahigh C-2 separation selectivities.