The present investigation was carried out to study the response of six Egyptian extra long staple cotton varieties and three grades from each to mercerization under tension treatment. Samples were spun carded and combed (20%) to produce yarns of 60's count at 3.6 twist factor. Carded and combed slivers were tested to determine fiber prop‌erties before and combed slivers were tested to determine fiber proper‌ties before spinning and mercerization. Mercerization under tension was carried out by caustic soda solution (NaOH) 20% w/w. The results showed significant differences in single yarn strength. The highest value of increase for carded yarns was obtained for Giza 45 variety grade FG (13.11%) while for combed yarns the highest value was obtained for Giza 76 variety grade FG (17.95%). Yarn elongation% decreased significantly after mercerization and the percentage of de‌crease ranged from -3.19 to -19.52%. It is concluded that the response of extra long staple Egyptian cottons to mercerization under tension is affected by carding and combing process as well as the cotton grade, thus these factors should be taken into consideration in the processing to produce high quality cotton products.