During his self-exile in New York, Antoine de Saint-Exupéry not only wrote and published The Little Prince, but also Letter to a Hostage in 1943. While the French aviator and writer dedicated the first book to his Jewish friend, Léon Werth, he also wrote the draft of the later based on a letter originally addressed to Werth. This letter was also intended to become the preface to Werth’s memoir called 33 Days, a chronicle of a-8-million-people exodus during the German invasion in 1940. Werth asked Saint-Exupéry to smuggle the manuscript of 33 Days to New York for him and to get it publish in America. For some unknown reason, 33 Days had not found a publisher until 50 years later. Tormented by his friend’s unsuccessful publishing project, Saint-Exupéry was also worrying about Werth’s safety in occupied France. To turn these unfortunate events into a positive one, he rewrote and expanded the original letter to Werth before publishing it firstly as a wartime booklet. Letter to a Hostage includes Saint-Exupéry’s interpretation of the essence of humanity after experiencing the solitude in Sahara and being captured by the Spanish armed anarchists. In fact, the six parts of the booklet can be read in line with the six parts of a discourse in Rhetorica ad Herennium, for Saint-Exupéry aptly used many rhetoric devices in it. This essay begins by providing background information about 33 Days and Letter to a Hostage, then by using classic rhetoric works by Aristotle, Cicero, Quintilian, etc. as references, it will... followed by the description of ‘the inventio’ in Letter to a Hostage the disposition and style of each part; some brief introduction and topic in each; the maneuvers of various tropes, figures of thought and figures of speech; and the methods of demonstration and refutation, such as examples and enthymemes, used by the author in order to conciliate the French with opposite opinions and to denounce the totalitarianism. Finally, by comparing Letter to a Hostage with some other works of Saint-Exupéry, we wish to understand the writer’s reminisce about Werth and France, while demonstrating how the writer voices his appeal to America for intervention in World War II and his attempt to save the “forty million” hostages through literary writing.