A formula is given for calculating minimum electron-phonon coupling S factor in mul-tiphonon transitions, this formula has the advantage of being independent of lattice vibration modes. The S factors have been evaluated for the multiphonon transitions among the multiplets 5F1,5F4,5F5,5G4,5G5 of Ho3+ ion in the LaF3 crystal. The values obtained for the S factor are around 0.05, which is in agreement with experimental results.