Vertically-aligned ZnO nanorods with different rod diameters of 50 nm, 110 nm and 240 nm were synthesized by a low-temperature wet-chemical method. Morphologies and structures of ZnO nanorods were examined by scanning electron microscopy and X-ray diffraction. Third-order optical nonlinearities such as nonlinear refraction and absorption were determined by closed- and open-aperture z-scan measurements at 815 nm using femtosecond laser pulses. Enhanced optical nonlinearities were observed with increasing rod diameters and aligned degrees of ZnO nanorods along the c-axis. Nonlinear response time, measured by using a time-resolved optical Kerr-gate technique, was less than 95 fs. (c) 2007 Elsevier B.V. All rights reserved.