The objective of the present study is to test and evaluate the performance of Mabrouk type threshing machine for threshing sunflower heads (variety of vidok) after modification of the threshing drum and the cleaning unit. The machine performance was evaluated in terms of threshing efficiency, percentage of un-threshed seeds, percentage of seed losses, seeds purity, fan losses and losses behind sieves. The experimental work was carried out to investigate the effects of some engineering parameters of the machine threshing and cleaning units included threshing drum speeds of 300, 400, 500 and 600 rpm (10.99, 14.65, 18.31 and 22.78 m/s) ,heads feeding rates of 300, 450, 600 and 750 kg/h, air velocity of 3.5, 4.5, and 5.5 m/s. suction fan, and sieve tilt angles of 2, 5 and 8 degree. The obtained results indicated that, considering the lower percentage of un-threshed and broken seeds and the higher threshing efficiency with acceptable machine capacity, the machine feed rate of 600 kg/h. and drum speed of 400 rpm is recommended for threshing sunflower heads. Meanwhile, considering both seed purity and total seed losses, it canbe said that the air velocity of 4.5 m/sec with sieves tilt angle of 5 degree recorded proper seed purity of 98.5 % with total seed losses of 0.42 %.