Osamu Dazai, a most important representative writer of the rogue faction in the Japanese literary, is regarded as the pioneer of modern Japanese literature. His master piece "No Longer Human" which is regarded as autobiography wind works as the Testament is the summary of Dazai’s life and artistic creation.People in China or Japan are studying Osamu Dazai and his works like "Osamu Dazai’s life, sin, death", "Osamu Dazai’s female image". However, few people research on Osamu Dazai’s human nature. This paper is to study the human nature of Osamu Dazai and its formation factors by analyzing his work "No Longer Human".The thesis consists of four parts. The first chapter analyzes the humanity of Osamu Dazai from human fears in his work "No Longer Human". the fear of terror begins the hero’s character. The hero, questioning about mankind and hardly finding ways to overcome his fear of terror, gets along with people by playing the clown.The first chapter analyzes Osamu Dazai’s human nature from his horror, ways of playing clown and other aspects. The second chapter is to analyze Osamu Dazai’s self-abandoned from his collective resenting and comfort of participating the illegal movement. The third chapter is to study his consciousness of the crime and punishment and the vision of death. The fourth chapter is the analysis of the formation of Osamu Dazai’s human nature from the family, society and the influence in Japanese literature.Osamu Dazai is a man of contradictions. His human nature has many characteristics. By analyzing his work “No Longer Human", we can see his real side of humanity 、powerlessness 、pessimism and suspicion of human nature.