Threshing losses (%), separating losses (%), shoe losses (%) and threshing efficiency (%) were calculated and determined in this investigation as the parameters required for evaluating the performance of the modified threshing device of a cereal crops harvester with the aim to reduce threshing and separating losses and reduce damaged seeds. The main characteristics of the used harvester were CICORIA -T type, longitudinal axial-flow with standard 5 bats reel. The modification was accomplished by replacing the wrapped rasp-bars by shaped prismatic beaters made from steel and fixed (bolted) tightly on the same (6) lines of the original rasp-bars. The factors considered to test and evaluate the performance were combine forward speed of 2.40, 2.82, 3.30, and3.80 km/h: seed moisture content of 22.35, 15.15 and 10.75(%wet basis): threshing rotor speed of 9.14, 10.43, 11.73 and 13.49 m/s and concave-rotor clearance ratio of 4.2/2.8and3.25/2.5 .The direction N  S (North-South) was used in sunflower seed crop harvesting as suggested in a previous study. The results revealed that the least values of threshing, separating and shoe losses were about (0.18% or 2.31 kg/fed), (0.14% or 1.8 kg/fed) and (0.11% or1.61 kg/fed) respectively at the forward speed of 3.3 km/h, clearance ratio of 4.2/2.8 and seed moisture content of the range from 10.75 to 15.15 % wet basis. Rotor speed of 13.49 m/s (379.27 rpm) realized the least values of both threshing and separating losses at the previous conditions, while the least values of shoe losses were achieved when the rotor speed decreased from 13.49 to 9.41 m/s at the same previous conditions. The lowest value of 0.73% processing losses (threshing, separating and shoe losses) of the total yield was occurred under 1.5 concave clearance ratio, rotor speed of 11.73 m/sec and 15.15% seed moisture content