  U_A(1) symmetry breaking is studied by introducing the flavor mixing
interaction proposed by Kobayashi, Maskawa and 't Hooft. Combining the one
gluon exchange interaction, the rainbow like Schwinger-Dyson equation and the
ladder like Bethe-Salpeter equation are derived. The anomalous PCAC relation in
the framework of this approximation is considered. The masses of the
pseudoscalar mesons pi,eta and eta' are calculated. It is found that the pion
mass is not sensitive to the strength of the flavor mixing interaction. On the
other hand, the masses of eta and eta' are reproduced by a relatively weak
flavor mixing interaction, for which the chiral symmetry breaking is dominantly
induced by the soft-gluon exchange interaction. The decay constants are
calculated and the anomalous PCAC relation is numerically checked. It is found
that the flavor structures of the eta and eta' mesons significantly depend on
their masses and therefore it is quetionable to define a flavor mixing angle
for eta and eta'.
