We first took advantage of a modified solvothermal route to achieve deliberate size modulation of ZnO nanorods. In this facile wet chemical process, the crystal sizes are sensitive to the surfactant concentrations, due to the efficient selecting adsorption of surfactant on the ZnO crystal planes. As a result, a fine size tuning of ZnO nanorods (from about 6.5 nm to about 120 nm) with high quality is conveniently achieved. The as-prepared ultrathin ZnO nanorods were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and x-ray diffraction. Optical properties of the ultrathin ZnO nanorods were also studied; the strong UV excitonic emission and weak deep-level emission reveal their very limited structural defects and narrow size distribution, which is interesting and important for facilitating their special applications for photoelectric nanodevices in modern science and technology.