Single-crystalline ZnO nanowire arrays with different aspect ratios and nanowire densities were prepared by the hydrothermal growing method using polyethyleneimine (PEI) as a surfactant.PEI can only hinder the lateral growth of the ZnO nanowires,which is observed by high resolution transmission electron microscopy (HRTEM) analysis.Dye-sensitized solar cells were assembled by the ZnO nanowire arrays with different thicknesses,which can be controlled by the growing time and characterized using photocurrent-voltage measurements.Their photocurrent densities and energy allover conversion efficiencies increased with increasing ZnO nanowire lengths.Short-circuit current density of 4.31 mA.cm-2 and allover energy conversion efficiency of 0.87％ were achieved with 12.9-μm-long ZnO nanowire arrays.