Nobuo OKISHIO (1927-2003) is famous as one of pioneers in the introduction of mathematical model analysis into Marxian economics. However, there is no common understanding about what is the essence of Okishio's economics. The purpose of this paper is to explore theoretical and methodological features of Okishio's economics through investigation of his works concerning the principles of capitalism. Naturally this task involves a critical review of some aspects in its accomplishments. Okishio's theory of the general principles of capitalism consists of four parts: reproduction structure, wage-price relation, short-term equilibrium, and capital accumulation. These four parts are further divided into two groups. The first two parts analyze the objective structure of capitalist production by Leontief type input-output models. The latter two parts deal with the mechanism which keeps the fluctuation of real wage rate within a certain range through business cycle. Here Okishio adopts the framework of Keynes-Harrod type macro model which presupposes the independence of the demand for investment. These four parts are mutually connected and each presupposes others. As a whole, they constitute a theoretical system explaining the reproduction of capitalist economy. His description of the reproduction of capitalist relations is completed through the whole of these four parts. Starting from the relation between physical quantities and labor hour, he critically and rigorously reconstructed Marx's theory of value and exploitation. By clarifying the concept of surplus labor, he succeeded to show internal linkage between categories in the world of commodity exchange and objective structure of reproduction. At the same time it is also possible to regard Okishio as one of the most penetrating internal critics of Marx's value theory. On the problem of wage-price relation, he elucidated the law of the uniform profit rate caused by the change of technology and real wage. This analysis is based on the assumption that the choice of technique by capitalists follows the cost-minimizing principle. Beyond the analysis of objective structure of capitalist economy, Okishio tried to build a coherent theory of business cycle as the process of cumulative disequilibrium by clarifying the peculiarity of the decision-making of capitalists concerning production, employment and investment. Considering that many Marxian economists had been indifferent to the problem of how individual capitalist behaved, Okishio's attempt was certainly a great challenge. However, his theory on this matter is unsatisfactory on several counts. His characterization of capitalist's supply behavior does not correspond to his formal model. His analysis of business cycle lacks explanation of the mechanism preventing the perpetuation of cumulative disequilibrium. In spite of Okishio's exceptional talent and passion, his endeavors to incorporate the theory of individual choice and behavior into the explanation of reproduction of capitalism did not reach the goal. Thus Okishio's economics has left us a difficult problem to combine structural and behavioral analysis into single coherent theoretical framework.