Motivated on knowledge representation and veterinary domain this project aims at using semantic technologies to develop a tool which supports veterinary diagnosis. For this purpose an ontology based inference engine was developed following the diagnosis task definition provided by CommonKADS methodology. OWL was the language used for representing the ontologies, they were built using Protégé and processed using the Jena API. The inference engine was tested with two different ontologies. This shows the versatility of the developed tool that can easily be used to diagnose different types of diseases. This is an example of the application of CommonKADS diagnosis template using ontologies. We are currently working to make diagnoses in other domains of knowledge.