The Knowledge.NET language we developed in our project is an extension of C# by hybrid ontology-based knowledge representation features. Ontology is a kind of specification of a problem domain in terms of its concepts and their relationships. As a basis of Knowledge.NET, we use our previous inventions - the knowledge representation languages C# Expert and Turbo Expert. Unlike a lot of conventional frame-based languages, Knowledge.NET is extended by constructs to represent ontology knowledge. The semantics of ontology in Knowledge .NET is similar to OWL. In Knowledge .NET we use Concept-Property-Individual set of terms (used in ontology-based systems), instead of Frame-Slot-Instance set of terms (used in frame-based systems), instead of Frame-Slot-Instance set of terms (used in frame-based systems)