The paper deals with ontology modelling for the purpose of design and creating of knowledge ontology for thematic cartography. The prepared ontology represents a database of selected declarative cartographical knowledge subsequently employed in the intelligent system for interactive support of thematic map design. The knowledge system pilot project under development is intended for users without necessary cartographical knowledge to whom such system will enable interactive creation of a thematic map and will provide them with support to this aim. The paper brings up information on possibilities of a domain expert (cartographer) in the endeavour to seize domain knowledge of thematic cartography and to express them in a formalized way. OWL ontology concentrates, formalizes and organizes declarative knowledge of thematic cartography domain. The result is the database containing taxonomy of terms hierarchically arranged into categories as well as description of their mutual relations.