Abstract: Tracing back the formation and perfection of the early quantum theory to the current quantum mechanics theory by combining the special historical background concerning the development of the quantum theory of the 20^th century,the article discusses the extraordinary work done by Einstein in the aspect of quantum mechanics is discussed. It is also pointed out that Einstein played an important role in different periods in the development of the quantum theory,thus producing an impact over the study of quantum theory and advancing the development of quantum mechanics.

Key words: Einstein, quantum theory