The vibration of Zinc Oxide (ZnO) nanowires is studied via molecular dynamics (MD) simulation and continuum theory. The size effect of equivalent Young′s modulus and piezoelectric constant for the ZnO nanowires are described by core-shell model. The equivalent tensile Young′s modulus of ZnO nanowires in polarization direction increases gradually with the increase of the cross section size. The equivalent tensile Young′s modulus predicted by continuum theory is in a good agreement with the MD result. The equivalent bending Young′s modulus also increases with the increasing cross section size. Meanwhile, the piezoelectric constants of ZnO nanowires are larger than that of piezoelectric ceramics. The piezoelectric constants of ZnO nanowires decrease with the rising of the cross section size. In addition, the vibration of the cantilevered nanobeam made of ZnO is simulated by MD. The vibration frequencies of a ZnO nanowire keep constant at different temperatures. When the cross section size becomes larger, the vibration frequencies predicted by continuum theory get closer to those obtained by MD well.