We conducted discontinuity logging and estimated Geological Strength Index (GSI) of the Neogene-Paleogene sedimentary and the Cretaceous igneous rock core samples taken for an offshore wind farm project off the Kyushu (Southwest) region in Japan. Bottom-fixed foundations, namely jacket type foundations are planned for wind turbines. Whilst there have been few cases where the GSI system is applied to onshore projects as well as offshore projects in Japan, we attempted to employ the GSI system to characterise the rock foundation for offshore wind farms instead of Japanese conventional rock mass classification system proposed by Central Research Institute of Electric Power Industry (CRIEPI). This study compares the GSI values estimated from different approaches with Rock Mass Rating (RMR) calculated based on Bieniawski (1989) and the CRIEPI rock mass class to discuss the applicability of the quantitative GSI system to offshore wind projects in Japan. The rock mass strength and deformation modulus were calculated from the estimated GSI values and unconfined compressive strengths (UCS) obtained by laboratory tests on intact rock based on the Hoek-Brown criterion. We propose a quantitative approach of GSI = 90/70 x (R2+R3+R4) - 5, modified from Song et al. (2020), where R2, R3 and R4 are the ratings for RQD, the spacing of discontinuities and the conditions of discontinuities respectively in the RMR system since it has a good correlation with qualitative GSI values. The GSI values generally range from 25 to 70, which are equivalent to the range of 35 to 80 in RMR values. The GSI values of each CRIEPI rock mass class generally agree with the typical ranges suggested by Cai et al. (2002); however, the estimated GSI values tend to be larger. Whilst equivalent Mohr-Coulomb friction angles derived based on Hoek et al. (2002) tally with a typical range presented by Yoshinaka et al. (1989), equivalent Mohr-Coulomb cohesive strengths vary in a wide range depending on UCS. The rock mass deformation moduli estimated based on Hoek and Diederichs (2006) correspond with those obtained by pressuremeter tests.