  By examining in details the disk level S-matrix element of one massless RR
and one NSNS states at order O(\alpha'^2), we find the coupling of one RR and
one NSNS fields on the world volume of a D$_p$-brane. The non-zero couplings
involve the first derivative of the RR field strengths F^{(p)}, F^{(p+2)} and
F^{(p+4)}. We then fix the on-shell ambiguity of the couplings by requiring
consistency with the linear T-duality transformations. Moreover, consistency
with the non-linear T-duality requires that the RR field strength in the above
couplings to be {\cal F}=d{\cal C} where {\cal C}=e^{B}C.
