This study was carried out to determine the processing variables i.e.. sliver variables, rotor speed and rotor diameter more appropriate for spinning Egyptian long staple cottons on the open-end rotor spinning machine in routine testing and quality evaluation of different cotton strains especially those included in the breeding programme carried out by the CRI. Giza 83 long-staple cotton variety was spun into 30s yarn count at 4.2 twist multiplier at various rotor speeds and rotor diameters and also, sliver variables. The essential findings of this study could be summarized as follows: 1. A significant improvement in lea count strength product, une-venness and yam imperfections occurs with the increase in the drawing passage. The improvement in yam properties is also greater in case of second draw frame passage compared to the first draw passage and card sliver. 2. A 31 mm-diameter rotor tended to produce stronger, more even and less yarn imperfections than a 33mm and 40mm-diameter ro‌tors. 3. Acceptable yarn quality was attained at a rotor diameter of 31mm. running at 100.000 rpm. 4. Therefore it is concluded that in the standard technique draw‌ing passage. 31mm-diameter rotor and 100,000 rpm. rotor speed are appropriate.