Abstract: An experiment was conducted to study the effect of High Ambient Temperature (HAT) on performance of four genotypes of broiler [Rose (RS); Cobb (CB); Hubbard (HB) and Lohmann (LN)]. A total of 700 day-old unsexed broiler chicks were used. Broiler chicks were obtained from four commercial parent stocks all bred in Iraq. Chicks (175 per genotype), 25 per pen, were housed in hall contained 28 pens (seven pens per genotype). Individual Body Weight (BW), in males and females, was determined at hatch and weekly then after at the end of experiment at 49 days of age. Feed Consumption (FC) and Conversion Ratio (FCR) and mortality percentage were determined per pen. All four genotypes were received starter diets (3005 Kcal/Kg feed; 22% CP) from one day to 28 days of age and grower diets (3059 Kcal/Kg feed; 20% CP) from 29 to 49 days of age. The genotype group differed significantly in BW and body weight gain (BWG), FC, FCR and mortality. The hot season effect was largest on BWG form 29-49 days of age along with reduction in FC and partially in FCR. The reduction in BW and BWG in both sexes (males and females) due to HAT appeared to be independent of sex. The greater reduction in FC and FCR was occurred in last period of study (42-49 days of age). This study suggested that standard broiler genotypes must be tested in our hot climates in summer season in order to find the one most suited to perform better in these conditions. Furthermore, it was prefer to marketing broilers at early ages (less than 42 days of age) to avoid the deteriorate effect of HAT on productive performance of broilers.