Reduced graphene oxide/zinc oxide (RGO/ZnO) nanocomposites were synthesized by facile hydrothermal method; this method consists of exfoliation of graphite with modified Hummer's method and synthesis of (RGO/ZnO) nanocomposites using hydrothermal method. X-ray diffraction, TEM and FT-IR spectroscopy were used for structure morphological characterization. Optical properties of the samples were studied by measuring their optical-absorbance. It can be concluded from XRD analysis that there are improvements of crystallinity associated with the increase of the crystallite size, as well as the increase in both of the (Zn-O) bond length and unit cell volume with the increase of ZnO ratio in RGO/ZnO nanocomposites. The morphological studies confirmed that the scale of ZnO particles is large and the size distribution is not uniform in samples of RGO/ZnO and that most particles have average size of about 9 nm while some particles even have larger size. The optical absorption spectra show that the excitonic peak of the as-prepared samples is red shifted from 268 nm to 376 nm for reduced grapheme oxide (RGO) with the increase of ZnO ratio, and the optical band gap changes from 2.39 for pure RGO to 3.34 eV for pure ZnO. The effect of UV irradiation on the optical absorption nanocomposites at different UV doses was carried out using UV lamp.