Due to population growth, limited water resources, fossil fuels and needs for agricultural production, the necessity of agricultural mechanization is increasing, especially in developing countries. The primary objective of the operating agricultural tractors, especially those in middle to high ranges of power, is producing of drawbar power. Since, the tractor engine and power transmission system efficiencies are almost constant and less affected by the tractor working conditions, any improvement in total efficiency of these tractors will be achieved by increasing the tractive efficiency. So, the most important purpose in designing farm tractors, especially those having the medium to high power ranges, is enhancing their maximum drawbar power (tractive power). In this study, the field performance of two middle power tractors, namely as the ITM 800 (a product of Iran-Tabriz tractor manufacturing company) and Sepahan 804 (an imported product of Sepahan tractor company), respectively having power of 82 and 80 hp was investigated during plowing with a 3-bottom Massey Ferguson mouldboard plow. The objectives of the present project included: 1) evaluation and comparing the power delivery efficiency, tractive efficiency, fuel consumption and specific fuel consumption of the tested tractors affected by the tractor type and tires ballasting, and 2) evaluation and comparing performance of the tractors in 2WD and 4WD modes for different levels of draft forces in moderate and deep plowing conditions. The experiments were performed in split-plot factorial in randomized complete block design with three replicates. Treatments included two tractors operated in 2WD and 4WD modes, both in two axle load situations (standard and ballasted axle load), and loaded in moderate and deep plowing conditions. Results indicated that, there were no significant difference in draft force for the tested tractors, but Sepahan 804 had more slip with respect to ITM 800 (16% in average). The average of fuel consumption for Sepahan 804 was 14% less than the corresponding value for ITM 800. However, regarding the specific fuel consumption, which is more valuable in comparing of tractors performance, no significant difference was observed. Although, the average tractive performance of ITM 800 was 5.6% more than one for Sepahan 804, there was no significant difference between them in power delivery efficiency. Ballasting reduced the average slip by 8.5%, and increased the draft force by 6%. Also, through ballasting the tractors, the average fuel consumption decreased by 8.5%, showing the necessity of suitable ballast in tillage operation, especially in deeper plowing. The 4WD mode obviously decreased the slip average (52%), and increased the average of draft (17.5%). Also, engaging the front axle increased the tractive efficiency by amount of 19.2%. In spite of 60% increase in the produced draft force, the average slip was increased 49%, as the plowing