This study was carried out at Shandaweel Agric. Res. Station (Sohag Governorate), Egypt during 2009 and 2010 seasons. At each season three planting date treatments (March 25, April 10 and April 25) were studied on the Egyption cotton Giza 90 cultivar. The experimental design was Randomized Complete Blocks (R.C.B.) with four replications. Obtained results revealed that plant ages 75, 105 and 135 days late sowing obtained the highest number of heat units and resulted in increasing plant height, number of nodes per plant, dry weight per plant, number of leaves per plant, leaf area and leaf area index while, it had no effect on internode length, leaf area index in age 135 days. Number of sympodia per plant increased in favour early sowing due to lower night temperature which lowered the position of early sympodium and inducing early balance between vegetative and fruiting development. Number of leaves, leaf area and leaf area index of early sowing increased slowly through, early season. Late sowing produced the first flower and open first boll in shorter period as compared to early sowing, but it consumed more number of heat units. The highest yield components and yield per unit area was produced from early sowing, where the cotton plants received the highest number of heat units through the growing season as compared to late sowing. Most of these units were utilized in produced the fruiting organs, while late sowing used most of these units in increasing the vegetative growth. The efficiency use of heat units by cotton plants increased in favour of early sowing rather than in late sowings. Best results were obtained when cotton plants were sown early (25 March). Generally, it is recommended to grow cotton early (25 March) for Giza 90 cotton cultivar gave the highest seed cotton yield per feddan rather than the late sowing (25 April) under the condition of this study.