Luminescence properties of highly (0001) oriented ZnO whiskers obtained by atmospheric chemical vapor deposition have been investigated over wide temperature range of 12 K to 300 K. An ultraviolet emission associated with bound-exciton recombination has been observed at 12 K under the low power photoexcitation. The emission exhibited temperature quenching with an activation energy of ∼9 meV, which is treated as the binding energy of the bound exciton. The shape of the emission was considerably modified at high-power UV laser excitation. The principal feature of this modification is an occurrence of a series of equally spaced, narrow emission components, decreasing in intensity at longer wavelength. The separation between these components is found to correspond to the energy of a longitudinal optical phonon (∼70 meV) determined by Raman scattering measurements. The activation energy of the temperature quenching of the emission considerably enhanced up to ∼58 meV under the laser excitation. This enhancement of the activation energy is discussed as being due to the domination of the free-exciton emission in the photoluminescense spectra under the high-power excitation. The balance between free- and bound-exciton states is discussed in terms of exciton generation and the annihilation scheme.