The article is devoted to the problem of the transformation of the genre gene survey of instrumental fantasy in musical creativity of the 20th century. The research methodology is based on cultural and systemic approaches, which allows to present fantasy in several dimensions: ontological (philosophical meaning), pragmatic (genre structure) and interpretive (play with an invariant, original work, etc.), which is proven on individual examples. The scientific novelty of the study consists in presenting fantasy as a system that combines the ontological, communicative and interpretive nature of the genre. It was revealed that the ontological nature is revealed through structural, linguistic and dramaturgical signs of improvisation and freedom; genre (genre composition) through the basic indicator that combines freedom of presentation and development of the main musical material (theme, thematic complex, etc.); the communicative meaning appears to direct the attention of both the performer and the listener to the principle of creative freedom, or a certain “game”, which becomes the main idea in the work. Finally, the interpretive meaning is emphasized due to the presence of the type of “fantasy on themes”, re-intonation, combination of the fantasy genre with other genres, which is often noted by the author himself - sonata-fantasia, waltz-fantasia, quasi fantasia, etc., which is evidence of the work of fantasy as a principle of special “world perception” and “world creation” both by itself and based on established linguistic and dramaturgical norms of another genre.