This study was conducted to determine the growth performance and carcass characteristics of two slow-growing broiler genotypes (Hubbard S757 and Hubbard Grey Barred JA) fed diets supplemented with dry oregano (Origanum vulgare L. or lemon balm leaves (Melissa officinalis L.) as growth promoter source under an organic housing system. In this study 240 chicks (mixed-sex) were allocated randomly into 4 experimental groups according to a 2 x 2 factorial arrangement for 2 broiler genotypes and 2 diets. The effects of dry herb leaves and genotype x herb leaves interaction on studied parameters were not significant at all weeks of age (P>0.05), except neck (%). Body weight, body weight gain, feed efficiency, carcass weight and yield, leg weight and yield, breast weight and yield, back weight and yield, edible giblets weight and yield of Hubbard S757 genotype were higher (P<0.05) than those of Hubbard Grey Barred JA genotype. The female breast (P<0.01) and edible giblets yield (P<0.05) were superior to those of males. These results show that herb leaves used as a growth promoter source under organic housing system did not affect the studied parameters and that in terms of these parameters, and subsequent Hubbard S757 genotype were superior to Hubbard Grey Barred JA genotype.