Using quantum chemistry ab initio method, geometry optimization of the singlet and triplet electronic states are performed for three isomers (ring, bowl and cage structures) of C20. From the optimized geometries, MP2 calculation gives a result which does not agree with Hund's rule, i.e. the singlet state has a lower energy for all the three isomers.Further more, total charge densities and electrostatic potentials of three isomers are given by the UHF wavefunctions,the results indicate negative charges located in the central area of cage and bowl structures, and the bond character of ring structure is analysed.