Combines and reapers harvest the grain crops, the green grass and other similar plants, therefore the cutter bar should be able to cut from 30 to 100 mm above the ground surface. The cutting parts should also be protected from hitting the rocks or the soil. It should also be considered that any crop remains of cutting height on the ground after the harvest are a loss. The muddy conditions found in rice growing area proved difficult, because the cutter could hit the ground when the combine header would sink into the soil. Running the stubble cutter into the soil would cause it to jam or partially plug up, making it ineffective. The main purpose of this study was to cut the crop at the lowest possible level by manufacturing an automatic control unit which controls upping/downing the combine header, in order to avoid the obstacles which face it when lowering the cutter bar level. An automatic control unit has been constructed locally at the engineering workshop of Rice Mechanization Center (R.M.C), Meet El-Deeba, Kafrelsheikh Governorate, Egypt during the year of 2006. The experiments were carried out during rice harvest season of 2007 in order to compare two combine systems for the combine (Yanmar-CA65V) under the same different operating conditions. The first combine has an automatic control unit (combine with control system). But the second combine hasn't an automatic control unit (combine without control system or the conventional combine). All experiments performed at the research farm of RiceMechanizationCenter. The obtained results may be summarized as follows: 1-     The optimum operating conditions for the combine with control system are at forward speed of 2.5 km/h and cutting height of 0.05 m.           ¹ Prof. and Head of Agric. Eng. Dep., Fac. of Agric., Kafr El-Sheikh Univ. ² Sen. Res. and Supervisor of Technical Office, Agric. Eng. Res. Inst., Egypt. ³ Engineer, Agric. Eng. Res. Inst., Egypt. 2-     The optimum operating conditions for the conventional combine are at forward speed of 2.5 km/h and cutting height of 0.10 m. 3-     The combine with control system is strongly recommended since it gives lower loss and costs, and higher field efficiency compared to the conventional combine.