Knowledge base is an important component of intelligent system and the main part of an expert system. Nowadays ontologies become a mainstream technology in knowledge base design. This technology helps to represent knowledge in the form of concepts, their properties, and axioms. Ontology is relevant not only for developers but also for managers, sponsors of the project, domain experts, and future users. Our analysis of successful ontology-based diagrams shows that visual sketching reveals the main concepts and the relations between them in informal way that helps to catch the main idea, the essence of the domain knowledge. The research analysis of the visual sketching method was performed using the semiotic approach. The paper contributes to the discussion on the influence of visual ontology diagrams on user’s perception and understanding. It highlights the features which allow making ontology conceivable and suitable for the better human interpretation and communication. It suggests considering aesthetic and ergonomic characteristics of ontology as supplements to countable ones.