Nowadays timely medical assistance is necessary for physicians/medical practitioners for decision making. Sometimes it is necessary for physicians to make decisions to diagnose the disease of a patient promptly. ‘Implementation of a Medical Decision Making Tool’, is a software tool was developed by implementing and incorporating efficient data mining techniques. In this project, with the help of WEKA software, which is a collection of classification algorithms for data analysis and feature selection, the decision making tool was implemented for the data preprocessing and classification on collected data sets of patients. Dataset for three kinds of diseases, heart disease, dermatology, and hepatitis were used to evaluate the performance of eight well-known classification algorithms. The area under receiver operating characteristic (ROC) and the accuracy were calculated for the selected algorithms and the best suitable algorithm for each disease was discussed.