Ground water is a vital source of fresh water in both urban and rural regions of the world. In Kerala, ground water meets the drinking needs of more than 60% population. Thrissur is the central district of Kerala, with a special well culture. Mostly each and every home has a dug well and ground water is the major source of drinking water in the district. Hence it is necessary to evaluate the ground water quality of Thrissur to ensure safe drinking water. Water quality index evaluation is an essential step to monitor, control and prevent the ground water pollution. The water quality index calculated for 56 observation wells of study area indicates the suitability of drinking water in each well. However there exist some uncertainties in the conventional method of water quality index evaluation. Fuzzy logic implemented in water quality index evaluation makes it more reliable in conveying the accurate status of ground water quality. Mamdani fuzzy inference system model with 10 inputs and single output was created for index evaluation. Trapezoidal membership function plots were assigned for both inputs and output and a total of 412 possible rules were created based on the water quality data and water quality standards. Fuzzy index reveals that 70% of drinking water in the study area belongs to excellent category, 16% belongs to good category and remaining 14% belongs to poor category that is unfit for drinking. According to fuzzy index Kodungallur, Keechery and Azhikode were the major ground water contamination sites identified in the study area. Relative root mean square error value of 1.2 % was obtained by comparing both index values. From the year 2015 to 2019 the percentage of poor water category has changed from 6% to 14% which indicates an increasing ground water contamination issue that needs immediate attention.