  We clarify the radion superfield dependence of 5D N=1 superspace action. The
radion is treated as a dynamical field and appears in the action with the
correct mode function. Our derivation is systematic and based on the
superconformal formulation of 5D supergravity. We can read off the couplings of
the dynamical radion superfield to the matter superfields from our result. The
correct radion mass can be obtained by calculating the radion potential from
our superspace action.
