This paper presents a simple petrographic approach to measure grain size distributions of fine-grained tight reservoir rocks, simultaneous to other organic petrographic measurements. Application of reflected light microscopy on polished blocks is a routine methodology for organic petrographic analyses including maceral examination and measurement of vitrinite reflectance. While routine petrographic analysis is in progress, the presented method enables operators to provide simultaneous reporting of quantitative grain size distribution by random orientation measurement of grain diameters using a relatively quick and simple procedure. This method provides added value to the routine analytical information provided by organic petrography laboratories. Application of ultra violet (UV) incident light under both oil and water immersion objectives provides a far better visual distinction of grain boundaries compared to white incident light. The improved visual distinction is likely due to differences in the diffused UV energy between surface scattering of the crystalline grains and the subsurface scattering of the incident light from internal irregularities such as grain boundaries and cemented areas in the polycrystalline rock samples. As a result of this optical phenomenon, grain boundaries are highly visible as a light blue UV rim. We suggest a random orientation measurement of 50-250 grain diameters, depending on sample sorting in various parts of a polished block, to give a valid statistical dataset and a valid size distribution histogram. (C) 2015 Elsevier Ltd. All rights reserved.