MedReader is a web application that employs the advance of ontology as its knowledgebase. Acting as an online article reader, MedReader benefits non-experts who are interested in reading online medical articles but have trouble comprehending all the technical terminologies appearing therein. In order to help its users mentally grasp the main idea of the article, the reader variegates technical terms that match the concepts in SNOMED-CT, a comprehensive medical ontology. Not only are the users informed a succinct textual definition of the medical terms, but key relationships between terms especially the is-a relationships can also be displayed at a mouse click. This paper describes MedReader, its architectural design and usability evaluation. With the rich coverage of technical terms in SNOMED-CT and the easy-to-use interface of MedReader, our evaluation demonstrates that the majority of users gain better understanding of medical articles when reading with the tool. © 2013 IEEE.