The 20 th century American literature is marked with self-reversal, in other words, writers created works in order to explore their inner world and themselves. Many authors turned to genre of fiction with helped them discuss and explain what was happening to the world and society in which they lived. This new direction in literature is known as modernism. Modernist writers focused on themes of individualism, social and cultural issues, such as racism, freedom, violence of life, etc. The ideas and themes of modernist literature remained drawn from the late 19 th century and authors presented their own perspectives on the same themes, but in the context of contemporary social problems. The movements that influenced the modernist literature are 19 th century realism and naturalism and their representation of the American society and people’s lives after the Civil War and World War I. The aim of the modernist writers was not only depiction of life “as it is”, but search of solutions to dilemmas and problems of the society of the 20 th century. The best example of influence of realism and naturalism is the 20 th century American fiction which brought new themes and characters into literature and these themes and characters are still explored in works of the 21 st century writers. In this paper, we are going to define the influence of the 19 th century realism and naturalism on the American fiction of the 20 th century through analysis of works by Ernest Hemingway and Mark Twain. The realistic movement of the 19 th century was focused on true depiction of life and current problems of the society. The writers tried to create realistic pictures of modern classes and manners of the American life while describing details that derived from their observation of events and personal experience. They attempted to show social process and compare them to general truth of existence. The development of realism as literary movement was a result of historical and cultural events in the American society. Fast rise of economy, immigration and cultural diversity greatly changed people’s life and attitude to the better world. Realism was focusing on criticism of those changes through comparison of evolving new American society and culture with changes that occurred in other countries. In their works, the realist writers focused on different values of people and different view on life in America; they also tried to explore all dimensions of motivation that encouraged people of different classes accomplish particular actions. Thus, the writers of the realism movement created the actual portray of the American society through factual description of what they saw and while adding their personal criticism in attempt to encourage people change their lives for better. Naturalism has the same premise as realism, however, as opposed to realist writers who merely described the world, naturalist writers focused on ugly side of social norms. Naturalist writers examined life as scientists examine nature; at the same time, their approach to life was pessimistic, they often depicted the process of destruction or degradation of society and human nature. In addition, they often used nature as a projection of social development, through symbolic depiction of nature phenomenon; they tried (as well as naturalists) inspire society for positive changes. Lit