The objectives of the present study were to construct and evaluate an automatic control system to be used for minimizing grain sorghum loss in straw. Experiments were conducted at Sakha Agriculture Research Station during the summer season of 2005 using grain sorghum (hybrid 102). Experiments were carried out using locally manufactured threshing machine. A gate was used to control the time of staying the threshed materials upon cleaning sieves. The effect of cylinder speed, holding time and feed rate on threshing performance (total seed damage, total seed losses, cleaning efficiency and specific energy) were measured. The construction and use of the automatic control system behind the cleaning sieves reduces grain losses in straw of about 78,80% compared with control treatment at cylinder speed of 26.18 m/s, feed rate of 5 kg/min and holding time 60 sec. In the same manner the highest value of cleaning efficiency was 96.35% at the holding time of 20 sec, cylinder speed 17.42 m/s and feed rate of 5 kg/min for the modified threshing machine. However, the lowest value of specific energy was 10.21 kW.h/Mg at cylinder speed 17.42 m/s, feed rate of 10 kg/min and holding time of 60 sse. The optimum operating conditions were holding time of 40 sec, cylinder speed of 23.61 m/s and feed rate of 7.5 kg/min for its good performance.