  In this paper, we study the manifestation of exact diffeomorphism in warped
extra dimension, with an emphasis on the Randall-Sundrum model. Utilizing the
covariant form of variation $\delta g_{MN} = \nabla_M \xi_N + \nabla_N \xi_M$,
we derive the nonlinear transformation rules for the metric fields in the
unitary gauge. As an off-shell symmetry, diffeomorphism governs the interaction
structure by connecting the neighboring orders of bulk action expansions. Our
analysis unveils that an on-shell diffeomorphism exists prior to radion
stabilization, but is broken by the Goldberger-Wise mechanism. This on-shell
symmetry enforces a mass-related equation $F'- A' G =0$ to eliminate one degree
of freedom, while ensuring the 5D action invariant up to a surface term.
Therefore, we conjecture that the on-shell diffeomorphism actually protects the
radion field from acquiring mass.
