Abstract: Rural banking slowly developed. The supervision bureaus repeatedly adjusted policies, but had only limited effect. To explain this phenomenon, The author introduced the opportunity cost, and analyzed the effect to the optimal scale of rural banking. On the base of the hypothesis of any microeconomicagency was rational economic man, by using the method of equilibrium analysis, the author built a mathematical model to solve the determiner of effect the optimal scale of rural bank. Then he found: in the transconformation economies and developing countries, because of inadequate market competition, returns on capital wasn’t convergence between industries, so the opportunity cost had the important effect on the enterprise scale; about setting up rural banking every microeconomicagency had different rational choice; because banking financial institutions had bigger opportunity cost, they lack the desire of large scale setting up rural banking: private capital was hard to investing, and objectively they had advantage of opportunity cost on becoming the main sponsor of the rural banking. In a word, the main sponsor system was badly in need of reform. But unless introducing the economic subject having the lower opportunity cost, the rural banking can’t have a large scale development.