With the publication of The Interpretation of Dreams in 1900, the theory of the unconscious was first introduced. Many psychoanalysts have attempted to further develop, enhance or modify his ideas and promote the birth of new theories. A significant figure in this crucial time of transformation and evolution of the major psychoanalytic theories was Melanie Klein. Using Freud’s psychosexual theory as a blue print, she formulated her well-known theory of object relations. This paper analyzes the development of the Kleinian theory across different time periods. Klein first attempted to support the infantile psychosexual theory and Oedipal Complex with clinical cases of children psychoanalytic therapy. The observation of the infantile fantasy and relationships to internal objects led to the birth of the object relations theory. Let us examine the theoretical development and main concepts of each stage of the process. In the beginning, Klein built her theory of unconscious fantasy and internal objects on the original libido theory by Freud. Then she attempted to explain the hostility and anxiety using the theory of death drive. Later she proposed two developmental theories; the paranoid- schizoid position and depressive position. In relation to these theories, she discussed the persecutory anxiety and depressive anxiety which may result from the processes of integration or splitting of the self and the ego. Towards the end of her life, Klein proposed the theories of love and reparation, jealousy and envy, and the psychological process of gratitude. Key words：M. Klein, Unconscious fantasy, Internal object, Paranoid-Schizoid Position, Depressive position, Persecutory anxiety, Projective Identification Keywords：unconscious fantasy, internal object, paranoid-schizoid position, depressive position, persecutary anxiety, projective identification.