Characterization of rock masses is one of the fundamental aspects of rock engineering. Particularly, as a rock mass characteristic, determination of the strength of closely jointed rock masses is difficult since the size of representative specimens including discontinuities is too large for laboratory testing. This difficulty can be overcome by using the Hoek-Brown empirical failure criterion in conjunction with the Geological Strength Index (GSI) Classification System. However, characterization of rock masses and determination of their strength may involve some uncertainties due to their complex nature. The fuzzy set theory is one of the tools to handle such uncertainties. This paper describes the application of fuzzy set theory to the GSI System by incorporating judgement and experience of practising engineers. For the purpose, the original GSI System and its modified form were defined by fuzzy sets, and Mamdani fuzzy algorithm was constructed Using 22 "if-then" rules for evaluating discontinuity parameters and their ratings considered in the GSI System. In addition, slope instabilities in heavily jointed rock masses selected from two open pit mines in Turkey were back analysed and the results were evaluated to demonstrate and to check the performance of this approach. (C) 2003 Published by Elsevier Science Ltd.