The geometrical structures and stabilities of (CO2)(n)(-) with the size range 3 less than or equal to n less than or equal to 6 are investigated by ab initio calculations including the effects of electron correlation. The calculations have shown that the structures of (CO2)(n)(-) can be formulated by either CO2- . (CO2)(n-1) or C2O4- . (CO2)(n-2), and that the geometry of the (CO2)(2)(-) dimer remains more or less in all the optimized structures. In all the sizes investigated in the present study, the most stable isomers are of the C2O4- . (CO2)(n-2) form, being consistent with the results obtained in photoelectron spectroscopic studies.