The intact rate of agricultural machinery means the degree of agricultural machinery in good condition. Its value influences directly on the degree of agricultural mechanization and the operational benefit of agricultural machinery. The economic nature of intact rate was researched with the principle of value engineering in this paper. The six forms of the ratio of effect to cost were proposed to display the operational benefit of agricultural machinery. As the ratio of effect to cost reached the maximal value the intact rate is called as economical intact rate. Based on the statistical data of Huanghai State Farm in Jiangsu Province of China from 1991 to 1997, the fitting between the ratio of effect to cost and the intact rate was completed with the method of Cubic polynomial curve fit. The optimal extent of economical intact rate under six forms was obtained. For this farm the optimal values of intact rate for the largemedium tractors and all agricultural machinery were 96.4 %～96.7 %,96.7 %～97.0 % respectively. By selecting agricultural machinery and arranging operational scheme according to economic intact rate, the maximal benefit of this farm can be realized.