-This article describes the development of medical expert system for diagnosis of human diseases [MExS]. MExS can be used to perform some statistical evaluations on patient's symptoms which we find in our daily life, to detect the particular disease. It is a decision support system based on the symptoms. We describe a new method to creating weighted fuzzy set rule to deal with the medical diagnosing problems from the training data. It is a causal rule (whose "IF" part truly causes the "THEN" part to happen as an effect) based medical diagnosis expert system. The researchers are try to advance to develop software that will assist the doctors, physician or a normal human being in making right decision without any consultation with the specialist. The aim of this paper is to develop a single expert which deals with the diseases of liver, kidney, ENT, MD, blood, nervous system, eye, and skin. This expert system is a collection of different specialists related to human into a single unit to predict the human diseases.