  The type IIA/IIB effective actions compactified on T^d are known to be
invariant under the T-duality group SO(d, d; Z) although the invariance of the
R-R sector is not so direct to see. Inspired by a work of Brace, Morariu and
Zumino,we introduce new potentials which are mixture of R-R potentials and the
NS-NS 2-form in order to make the invariant structure of R-R sector more
transparent. We give a simple proof that if these new potentials transform as a
Majorana-Weyl spinor of SO(d, d; Z), the effective actions are indeed invariant
under the T-duality group. The argument is made in such a way that it can apply
to Kaluza-Klein forms of arbitrary degree. We also demonstrate that these new
fields simplify all the expressions including the Chern-Simons term.
