We performed friction experiments on granite gouge under hydrothermal conditions to investigate the roles of the various initial grain size on frictional properties of granitic faults in the continental crust. In this dataset, we report raw data and processed data after correction from the ring shear tests. In the same directory in which this documentation is located, 9 zip files are contained. Each zip file contains the following several text files that can be opened via visual studio code. The zip file named Uxxx (e.g. U973) is the experiment data of Ring shear tests. There are three text files in each zip file. Uxxx-T-σ_n^eff-Pf -f/c (e.g. U973-(25-300)-100-100-f) An Excel file with raw data (11 columns) continuously logged and created during an experiment. ‘f’ represents fine-grained, and ‘c’ represents coarse-grained. Uxxx_corrected (e.g. U973_corrected) A text file with 5 columns after processing the raw data in (a). The file has the following information Time [s] Shear displacement [mm] Effective normal stress [MPa] Friction coefficient [-] Shear stress [MPa]