Abstract: To realize the development of agricultural modernization, it is necessary to promote the moderate scale operation of agriculture. The practice showed that the implementation of agricultural scale operation was to obtain the benefits not only from the returns to scale effect, but also from the market power effect. Applying a Cobb-Douglas function, this paper established an functional relationship between yield and price increment, developed a concept of yield elasticity coefficient of price increment, and constructed a theoretical model of agricultural scale economies with market power effect by dividing agricultural scale economies into three stages, including “returns to scale effect stage”, “returns to scale effect + market power effect stage”, and “market power effect stage”. Furthermore, this paper also tested the market power effect of agricultural scale based on a survey data. Results showed that: 1) Compared with labor force, land and capital have more significant effects on agricultural scale economies; and 2) The market power effect of ordinary farms was weak, which was in the stage of “returns to scale effect”, while the market power effect of large scale farms was strong because the returns to scale effect was still positive and it was in the stage of “returns to scale effect + market power effect”.