  We study the consistency of the ladder approximation and the rainbow
approximation of the Dyson-Schwinger equation of QCD. By considering the
non-Abelian property of QCD, we show that the QED-type Ward-Takahashi identity
is not required for the rainbow-ladder approximation of QCD. It indicates that
there does not exists any internal inconsistency in the usual rainbow-ladder
approximation of QCD. In addition, we propose an modified ladder approximation
which guarantees the Slavnov-Taylor identity for the quark-gluon vertex
omitting the ghost effect in the approximation.
