Afield trail was conducted at Sakha Agricultural research station farm, Kafr El-Sheikh governorate to study  the effect of Ethrel at low concentration ( 5 cm / litre) as foliar application at different stages of bolls maturity (35, 50, 65 and 80 % ) after 120 days after sowing  in addition of control ( untreated plants) on yield and its parmuters of cotton  Giza 97 cultivar during 2019 and 2020 seasons.Complete randomized block design was applied with three replicates were used with five treatments. There are significant differences between all treatments by using Ethrel  at the most of studie d charecters. Foliar application of Ethrel with 5cm/litre at 50% bolls maturity significantly increased  number of fruiting branches, total chlorophyll, number of open bolls, earliness percentage, lint percentage, boll weight and seed cotton yield / fed. On the other hand, spraying cotton plants with Ethrel in this stage optained the lowest values of boll retention %, boll shedding in opposite of untreated plants which obtained the highest values. There are not any significant differences between all treatment in some characters as plant height and seed index which not affected by the application of Ethrel. To further improve the boll opening percentage for better yield performance same experiment was conducted using 5 cm / litre of ethrel  at 50 % maturity at  four times, 10 days between them ( beginning with120,130,140 and 150 DAS) .