Background & Objectives: Ground water is one of the important water resources. Quality indices are amongst the comprehensive and routine methods to evaluate water quality. Fuzzy Logic, as an appropriate method, applies artificial intelligence to develop complex and unknown systems e.g. environmental indices. This study aimed to provide a comprehensive index based on fuzzy logic and qualitative properties of ground water. Methods: Considering their effects on water quality and human health, 24 physico- chemical parameters were used for this study. Qualitative index was designed based on Mamdani inference system and tested using the data obtained from aqueducts located within Yazd city. Results: Fuzzy groundwater quality index provides more strict outputs than the NSF which uses nine parameters to present water quality. The differences observed might be due to differences in the number and interrelationships between parameters used for each index. Assessment of Yazd’s aqueducts water quality using Fuzzy index indicated that ground water quality is lower than the standard values set for drinking purposes. Conclusion: According to the results obtained, the proposed index can be used as a comprehensive tool to present actual water quality, especially for drinking purposes.