Harvesting is one of the most important field operations in sunflower production. The mechanized harvesting methods can induce a large percentage of damage, as large as 46%, to the seeds during threshing operation in a grain combine. In Iran, harvesting is often done manually. In this study, we redesigned the threshing unit of a grain combine in order to minimize the damage and maximize the threshing efficiency during harvesting operation. Mechanized harvesting of any crop by a grain combine consist of: collecting, threshing, separating and clearing operations. The threshing is the most important operation in a combine; because any disturbance in this unit makes some undesirable performances such as reduction in harvesting efficiency, increase in crop damage and also causes disturbances in other sections of the combine. To study the effect of some parameters on the threshing operation, a threshing cylinder and concave were designed and developed in laboratory scale. Variable parameters were the cylinder speed, the distance between the threshing cylinder and concave, the feed rate and the moisture content of sunflower head. A constant feed rate was selected by changing the combine speed considering the direct relationship between the feed rate and the sunflower plant density and combine speed. The tests were performed at 3 cylinder speed levels (280, 380 and 480 rpm), 2 distances between threshing cylinder and concave (8, 10 cm) and 2 moisture contents of sunflower head proportional to crop condition (20 and 45% wet basis). Then, the values of power consumption, separation efficiency of the concave, threshing efficiency and seed damages were analyzed. Azargol variety was used to evaluate the threshing cylinder unit. Prior to the evaluation test, the physical properties, the percentage of grain mass on sunflower head and the percentage of unfilled seeds were determined. To determine the effect of impact or scrubbing on the threshing sunflower at the different moisture contents, the concave was divided into 5 sections and the seed of each part was colleted by a pan. The results indicated that the impact was more effective on threshing operation at 20% moisture content, whereas at 45% moisture content the scrubbing was more effective on seed removal from the head. The results also showed that the stresses applied to the seeds during threshing operation were less than the rupture stresses of the seeds. The analysis of variance showed that the effect of the cylinder speed on the power requirement, the separation efficiency of the concave and the threshing efficiency were significant at 1% level. The effect of moisture content and the distance between the threshing cylinder and concave on threshing efficiency was not significant. But the effect of moisture content and distance between the threshing cylinder and the concave on separation efficiency of the concave were significant at the 5% level. The effect of moisture content on the power requirement was significant at the 1% level and also the power requirement was affected by the distance between threshing cylinder and the concave at the 5% level of significance. Keywords: harvesting sunflower, threshing cylinder and concave, physical properties of sunflower head