Generally, physician will question the patient about chief complaint and other symptoms. He will diagnose body of patient, and he will send the patient to laboratory for careful diagnosis. However this research dose not considers the result from laboratory. So when we consider the process for diagnosis of physician, we will find 3 important components [3]. These components are problems of patient, data, and diagnosis. The diagnosis is an important condition for indicating accuracy. So design of inference engine of knowledge-based system must consider the process for diagnosis of physician, it is shown in figure 9. The inference engine of this knowledge-based system is interactive forward chaining (show in figure 10). It uses facts of symptom and indication of disease for definition direction of inference. The process of interactive forward chaining inference in the medical knowledge-based system as figure 9. The system first obtains chief complaint from the patient and places it in the blackboard. The inference engine then scans the rules in some predefined sequence looking for one whose premises match the contents in the blackboard or query the patient. If it finds a rule, it adds the rule’s conclusion to the blackboard, firing the rule, then cycles and checks the rules again looking for new matches. On new cycle, rule that previously fired are ignored. This process continues until no matches are found. At this point the blackboard contains facts supplied by the patient and inferred by the system. To illustrate this process of interactive forward chaining consider the following medical rulebase