Influence of the cavity size on the response of LiF TLD was theoretically investigated for a presumed system of spherical TLD cavity imbedded in a medium of polyethylene. Calculation of the response for different radii of the spherical cavity was carried out as a function of incident photon energy, applying recent cavity theory. The range of the radii covers 1.578 to 6.528 mm, while that of the incident photon energies extends from 0.02 to 3.0 MeV. As a results, the response of the LiF TLD imbedded in a medium as a cavity was found to be functions of its own size as wall as the incident photon energy.