Phonon dispersion relations in Cu2O have been studied at 20°C using inelastic neutron scattering. Seven acoustic branches and twelve optical branches have been studied in detail in the three symmetry directions [00ζ], [ζζ0], and [ζζζ] of the cubic lattice. Four of the six zone-center phonons have been observed and the assignments and energies are confirmed as Γ25=87±2 cm-1, Γ12′=105±3 cm-1, Γ15=146±1 cm-1, and Γ2′≈347 cm-1. The dispersion relations agree only qualitatively with the rigid-ion-model calculations. It is suggested that more detailed calculations may be performed in the light of the present measurements. The elastic constants obtained from the slopes of the acoustic branches are (units of 1011 dyn/cm2) c11=12.61±0.12, c44=1.36±0.04, and c12=10.86±1.14, which are in close agreement with those obtained by the sound-echo technique.