Well-dispersed, high-aspect-ratio 4ZnO·B2O3·H2O whiskers (4ZnO·B2O3·H2OHARw) were synthesized by a facile hydrothermal method with the aid of ZnSO4·7H2O. The formation mechanism was investigated, and the results showed that Zn2+ and SO42− ions played a crucial role in the control of 4ZnO·B2O3·H2OHARw extended along [010] direction. X-ray diffraction (XRD) analysis, high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), and thermogravimetry (TG)-differential scanning calorimetry (DSC) results confirmed that the powder was 4ZnO·B2O3·H2O. Scanning electron microscopy (SEM) and TEM showed that the average aspect ratio of the product was about 100. Nanocomposites of surface-modified 4ZnO·B2O3·H2OSM-HARw/PP were prepared and their mechanical properties measured. The results suggested that the mechanical properties of 4ZnO·B2O3·H2OSM-HARw/PP composites were superior to composites made with low-aspect-ratio whiskers (4ZnO·B2O3·H2OSM-LARw), which displayed poorer mechanical properties even at low addition. [ABSTRACT FROM AUTHOR]