  Direct photon production at large transverse momentum in the coherent
diffractive processes at hadron colliders is calculated in the two-gluon
exchange model. We find that the amplitude for the production process is
related to the differential off-diagonal gluon distribution function in the
proton. We estimate the production rate at the Fermilab Tevatron by
approximately using the usual gluon distribution function. Because of the clean
signature, this process can be used to detailed study the small-x physics, and
the coherent diffractive processes at hadron colliders.
