Abstract: John Maynard Keynes is one of great monetary economists and his main works of economics are explanation of function of modern capitalist economies from a point of view of money. Final goal of Keynes’ three main books of A Tract on Monetary Reform, A Treatise on Money and The General Theory of Employment, Interest and Money are exploring the fundamental causes of business cycles in modern capitalist economies. The first section of this article gives a brief review of the states of studies of Keynes’ theories. The second offers some reviews of Keynes’ early theory on money and business cycles from his works in the early stage to A Tract on Monetary Reform, find that Keynes believes that managed currency is inevitable trend for a modern economy in the 1920s. Keynes also find that price level in one country is determined by banks’ credit and loan, and the credit and loan of commercial banks can endogenously create money. The third give some reviews on total theoretical structure and explanation of Keynes’ A Treatise on Money. In the beginning of the book, Keynes explores the various concepts of money in detail and creatively coins the two concepts of money, i.e., money of account and bank money. In the third part of Treatise, Keynes preliminarily constructs his theoretical framework of macroeconomic analysis. In the book, Keynes adopts Knut Wicksell’s concept of natural rate of interest but defines it as the one when saving equals to investment in a society. He further investigates the causes and mechanisms of business cycles in modern market economies from a point of view of deviation of market interest rate from the natural rate. The four section reviews the whole theoretical framework and demonstration logic of Keynes’ The General Theory. In the stage of The General Theory, Keynes has abandoned the Wicksellian concept of natural rate of interest and consequently creates his own concept of the marginal efficiency of capital. Based on the explanation of relations between marginal efficiency of capital and market rate, Keynes constructs his theoretical framework of industrial fluctuations of and finally attribute the fundamental causes of economic recession to a sudden collapse of the marginal efficiency of capital in an economy. The final section briefly gives some introduction of criticisms of Keynes’ economic theory of depression after the publication of The General Theory. Finally, we find that since most contemporary macroeconomists have abandoned Keynes’s concept of the marginal efficiency of capital, we can say that Keynes’ own theory of business cycles has be discarded by most mainstream economists today.