  With the LHC discovery of a 125 GeV Higgs-like boson, we study gravitational
interaction of the Higgs boson via the unique dimension-4 operator involving
Higgs doublet and scalar curvature, $\,\xi H^\dag H R\,$, with nonminimal
coupling $\,\xi\,$. This Higgs portal term can be transformed away in Einstein
frame and induces gauge-invariant effective interactions in the Higgs sector.
We study the weak boson scattering in Einstein frame, and explicitly
demonstrate the longitudinal-Goldstone boson equivalence theorem in the
presence of $\,\xi\,$ coupling. With these, we derive unitarity bound on the
Higgs gravitational coupling $\,\xi\,$ in Einstein frame, which is stronger
than that inferred from the current LHC Higgs measurements. We further study
$\xi$-dependent weak boson scattering cross sections at TeV scale, and propose
a new LHC probe of the Higgs-gravity coupling $\,\xi\,$ via weak boson
scattering experiments.
