Ontologies are relatively a new way of defining and storing knowledge and it describes a certain domain by dividing it into several concepts. It describes the relations between those concepts and finds many applications in all fields including disease diagnosis. Patients are diagnosed by medical doctors based on the signs and symptoms of the disease which may sometimes be misinterpreted due to lack of correct understanding/interpretation of information resulting in wrong treatment. In this paper, a computational model based on ontology is presented to reduce brain tumour misdiagnosis thereby making correct treatment decisions. This ontological representation provides structured knowledge in machine understandable form which can be shared across multiple platforms and uses a common vocabulary thereby helping in proper treatment