This paper shows the important role that plays the electron-lattice coupling to represent correctly the energy levels of Cr3+ ions in Nb5+ site into congruent LiNbO3 crystals doped with 5.3% of ZnO. Racah's parameters: B = 646 cm-1, 1; C = 3022 cm-1 and crystal field intensity y Dq =1342 cm-1 were determined and the Tanabe–Sugano's diagram was constructed. The characteristics of the absorption and emission spectra of Cr3+ ions in Nb5+ site have been explained in terms of the Configurational Co-ordinate model in the harmonic approximation. Huang–Rhys parameter, S=3.5 and the breathing phonon energy, ℏω = 460 cm-1 are also reported in this work. Different values of breathing phonon for Cr3+ in Nb5+ site than in Li+ site could explain the higher luminescent quantum efficiency of Cr3+ ions located in Nb5+ site in LiNbO3 crystals.