  We calculate the tree-level open-string amplitudes for the scattering of four
massless particles with diphoton final states. These amplitudes are required to
reproduce those of standard model at the tree level in the low energy limit.
After low energy stringy corrections, we found that they have similar form to
the same processes induced by exchange of the Kaluza-Klein(KK) excitations of
graviton in ADD scenario. Using this similarity, we apply constraints on the KK
mass scale $M_D$ to the string scale $M_S$. The results are consistent with
constraints from the 4-fermion scattering, about $0.6-0.9$ TeV.
