Annealing treatment was applied to different mesoporous ZnO nanostructures prepared by wet chemical synthesis, i.e. nanoflowers(NFs), spherical aggregates (SPs), and nanorods (NRs). The sub-bandgap,defect properties and the trapping state characteristics after annealingwere characterized spectroscopically, including ultrasensitivephotothermal deflection spectroscopy (PDS), photoluminescence and photoelectrochemicalmethods. The comprehensive experimental analysis revealsthat annealing alters both the bandgap and the sub-bandgap. The defectconcentration and the density of surface traps in the ZnO nanostructuresare suppressed upon annealing as deduced from photoluminescence and opencircuitvoltage decay analysis. The photo-electrochemical investigationsreveal that the surface traps dominate the near conduction band edge ofZnO and, hence, lead to high recombination rates when used in dyesensitized solar cells. The density of bulk traps in ZnO SPs is higherthan that in ZnO NFs and ZnO NRs and promotes lower recombination lossbetween photoinjected electrons with the electrolyte-oxidized species onthe surface. The highest power conversion efficiency of ZnO NF-, ZnO SPs-, and ZnO NR-based DSSC obtained in our system is 2.0, 4.5, and 1.8%,respectively.