ZnO nanopowders were successfully synthesized by ice-bath assisted sonochemical method and annealed in air for 3 h at different temperatures (Ta) range from 300 to 700 oC by step 100 oC. The effect of Ta on the structural and morphological changes was investigated by x-ray diffraction (XRD) and transmission electron microscopy (TEM). Optical properties were studied by measuring the optical absorption and photoluminescence (PL) spectra. XRD analysis showed that the thermal annealing leads to an improvement in crystallinity associated with the increase of the crystallite size, as well as the increase in both of the Zn-O bond length and unit cell volume. Also, it was found that increasing Ta results in a shift of the diffraction angle toward the lower values accompanied with the decrease of internal local strain. The morphological study confirms that the samples have a mixture of the nanosheets and nanorods structures. In addition, the length and diameter of the nanorods increase as a result of increasing Ta. The optical absorption spectra show that the exciton peak of the as-prepared sample is red shifted from 370 to 378 nm by the thermal annealing, and the optical band gap decreases from 3.45 to 3.36 eV. PL spectra were reordered at an excitation wavelength of 325 nm, and the deconvolution of the spectra reveals four emission bands; where the main UV band (at λ = 397 nm) can be attributed to exciton recombination related to near-band-edge. Furthermore, thermal annealing results in the quenching of PL intensity of the annealed samples.