Abstract: The object of this study is the analysis of the genre of the novel "The Heart of a Young Man" by the Thai king Rama VI Vachiravudh (1881-1925) – the monarch who stood at the origins of the formation of modern national literature of Siam / Thailand. The subject of the evaluation was the compositional and stylistic features of the work of the king-composer. Vachiravudh's innovation in the field of genre allowed not only to truthfully depict the personal experiences of the protagonist of the work, but at the same time to pose an important problem – man and society in an era of global change. Biographical, historical and comparative methods were used for a detailed analysis of the novel's genre originality. This is the first original Thai-language epic work of the monarch, which is of unconditional interest and artistic value. Previously, the novel was not translated into Western languages and did not become the subject of a separate study by both national Thai literary critics and foreign ones. In "The Heart of a Young Man", Vachiravud proved himself as a talented artist-innovator, creating an original genre synthesis: a sentimental novel in letters with elements of a realistic psychological novel. The form of the Western European sentimental novel in the letters of the XVIII century. Vachiravudh uses the connection of the social into the personal in the fate of an individual to express topical issues. Based on the material of "The Heart of a young Man", the plot and compositional organization of the epistolary novel, the originality of the typological features of the genre (Preface, indication of the place and date of writing, address to the addressee, signature, etc.), its chronotope and the relationship between the hero - narrator – author, as well as the features of the image of the inner world of the protagonist and his spiritual evolution are investigated. It is shown that, having inherited the tradition of European literature of the XVIII – XIX centuries, Vachiravud anticipated in his novel one of the dominant lines of development of Thai literature of the twentieth century.