Effect of particle size in aggregates on ZnO-aggregate-based dye-sensitized solar cells is investigated. A two-step hydrothermal method is developed for preparing submicrometer ZnO aggregates with different crystal sizes via controlling regrowth temperature. Three groups of ZnO-aggregate-based dye-sensitized solar cells with the different crystal sizes in the aggregates are fabricated for comparison of the photovoltaic properties. The results indicate that the cell made of crystal size of 25-30 nm has the highest light-to-electricity conversion efficiency of 4.54% for the dye-sensitized solar cells. According to the analysis of absorption spectra, dark current curves, photoelectron decay and electrochemical impedance spectra, we find that the aggregates with smaller crystal size have higher capability of dye adsorption, while the aggregates with larger crystal size have faster electron diffusion, and less recombination. Therefore, optimal crystal size in the aggregates for photoanode leads to balance these parameters to get higher light-to-electricity conversion efficiency. This investigation is important to the improvement of conversion efficiency for dye-sensitized solar cells. (C) 2013 Elsevier Ltd. All rights reserved.