Vertically aligned ZnO nanorods were successfully grown on porous silicon (PS) substrates by chemical bath deposition at a low temperature. X-ray diffraction, field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and photoluminescence (PL) analyses were carried out to investigate the effect of growth duration (2h to 8h) on the optical and structural properties of the aligned ZnO nanorods. Strong and sharp ZnO (002) peaks of the ZnO nanorods proved that the aligned ZnO nanorods were preferentially fabricated along the c-axis of the hexagonal wurtzite structure. FESEM images demonstrated that the ZnO nanorod arrays were well aligned along the c-axis and perpendicular to the PS substrates regardless of the growth duration. The TEM image showed that the top surfaces of the ZnO nanorods were round with a smooth curvature. PL spectra demonstrated that the ZnO nanorods grown for 5h exhibited the sharpest and most intense PL peaks within the ultraviolet range among all samples.