Abstract:  The paper aims at analyzing the authenticity of documentary which has long been discussed but remained controversial. By defining fiction, nonfiction and film of presumptive assertion, the paper summarizes the range and methods of defining the authenticity of documentary. Concepts of presentation and representation are distinguished in the first place. They cannot be discerned by external features of texts such as style or media form. Even though presentation and representation can be discerned, they cannot be used to differentiate fiction and nonfiction. Just as the concept of documentary proposed by John Grieson, there is presentation in nonfiction and representation in fiction. The paper holds that only when the audience recognize the producer′s intension and make presumptive imagination, can fiction film be defined, and nonfiction can be defined vice verse. In view of this, documentary is film of presumptive assertions, producer′s intension and message resource index being an important basis of presumptive assertion.

Key words: fiction film, nonfiction film, Film of presumptive assertion, documentary