We report preliminary results of the high resolution inelastic neutron scattering measurements of the 42.5 meV optical phonon branch in YBCO, whose behavior at momentum transfer q=0 has been extensively studied by Raman scattering. The experiment was done on a large (75g) single crystal of YBa2Cu3O7-x (T-c approximate to 90K) with the resolution of 2 meV (full width at half maximum). In cooling from 100K to 50K we observe a small softening of the phonon energy at q=0.25,0.25,0, but no significant linewidth change.