The sizedependence of photoluminescence (PL) of ZnO nanoparticles with diameter from 17 to 300 nm was demonstrated by time resolved laser spectroscopy. Broad PL spectra were obtained and found to consist of three Gaussian components. The authors found that the PL peak position and relative amplitude depend on the size of nanoparticles for the Gaussian PL band inside the energy bandgap. The Gaussian band Xc3 is believed to be associated with the transitions between surface states. It is well known that as the dimensions of a semiconductor are reduced to the nanometer scale, one of the key features is the large surfacetovolume ratio. The larger surfacetovolume ratio in nanostructures means the larger bulk density of dangling bonds. The existence of dangling bond in a crystal surface is likely to change a state localized by splitting the state out of the border of the energy gap. The Gaussian peak Xc3 lying inside the band gap of ZnO indicates the existence of such initial states in the forbidden bulk band gap. Furthermore, we found that with the decrease in particle size, the Gaussian curve Xc3 shifts to the lower energy. With the particle size increaseing, the comparative amplitude of PL band Xc3 descends rapidly and so does the corresponding relative PL intensity. The sensitive correlation between the particle size and the optical properties of the below gap broadband shows a key role of the surface states recombination in PL of nanosized particles. Furthermore, our exploration indicates that the surface states recombination plays a dominant role in PL of nanostructures with particle sizes down to a certain degree.