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Zinc oxide nanowires are considered as promising materials for wide range of optoelectrical and chemical devices, thanks to their desirable structural and optoelectrical properties. Over the past decade, chemical bath deposition (CBD) has been widely used to synthesize these nanostructures due to its low cost and controllability. Since improving the aspect ratio and length of nanowires is a vital issue in growing one-dimensional nanostructures, the influence of polyethyleneimine (PEI) as a complexing and chelating agent on the structural, morphological, and optoelectrical properties of ZnO nanowires has been studied in this report. As-grown ZnO nanowires synthesized by mixing deionized water, zinc acetate dihydrate, hexamethylenetetramine, and PEI were characterized with field emission scanning electron microscope (FESEM), X-ray diffractometer (XRD), and photoluminescence spectroscopy (PL). FESEM results unambiguously show that increasing PEI concentration (from 0 to 0.2 g in 50 ml DI water) reduces the diameter and density of nanowires from ~120 to 56 nm and from ~85% to 65%, respectively. Interestingly, although adding more PEI decreases nanowires diameter, over-increasing of PEI brings about an inappropriate nanostructures growth. Moreover, XRD patterns demonstrate that all the samples have wurtzite structure with a preferred orientation along c-axis which may be improved or deteriorated by adding PEI into the chemical bath. Accordingly, it is crucial to optimize the amount of PEI in CBD method. Near-band edge (NBE) region in PL spectrum also confirms wide bandgap of ZnO (~3.3 eV). In addition, comparing the appearance of PEI free with PEI assisted solutions show a considerable difference in their colors, which may be attributed to the formation of new chemical compounds. Considering these results, PEI plays a couple of determining roles in synthesizing ZnO nanowires; making nanowires thinner, with selectively absorption to the non-polar, lateral facets of wurtzite lattice, and controlling deposition rate by forming the PEI-Zn2+-HCHO complex compounds. [ABSTRACT FROM AUTHOR]