In this study, an expert system named ESMEF (Expert System for Mechanized Entrusted Farming) was developed. The function of ESMEF is to provide the various data and informations for entrusted farming such as farm machinery management data, mechanization systems by farm sizes, number of units and sizes of machinery needed, machinery replacement analysis, mechanization costs analysis. Mechanization systems were selected by ESMEF for different farming sizes of Chungnam Province and an economic analysis was conducted as an example. The results showed that the farm machinery purchasing costs were 1,344~4,829 thousand won per ha and there was no significant difference for farm sizes above 60 ha. The total annual machinery costs were 3,595~4,537 thousand won per ha, and a minimum cost was appeared for farm size of l00ha at first. According to this analysis, an optimum entrusted farming size would be 100ha by the present available farm machinery systems.