A study was conducted to determine the performance, carcase and gastrointestinal tract (gut) characteristics of broilers fed diets supplemented with dry peppermint (Mentha piperita L.) or thyme (Thymus vulgaris L.) leaves, which are among the alternative growth promoters. In the study, 312 one-week-old broilers (Ross-308) were used. There were 3 dietary treatments, each consisting of 4 replications (13 males and 13 females in each replication). The control group was fed basal diet, while the peppermint and thyme groups were fed diets supplemented with 0.2% peppermint or thyme (w/w) as menthol and thymol (70 mg per kg diet) sources, respectively. From 7 days to 35 days of age, the body weight gain was higher (P < 0.05) in broilers fed the peppermint-supplemented diet compared to the control, but the effect of peppermint on body weight gain disappeared at 42 days of age. Feed intake, feed to gain ratio, carcase weight, carcase yield, and the relative weights of the edible inner organs and whole gut, and the relative length of the whole gut were not significantly affected by peppermint and thyme contents. The peppermint or thyme leaves increased (P < 0.05) the abdominal fat pad at 42 days of age. Thus, the dry peppermint leaves had a higher growth promoting efficacy than the dry thyme leaves at an early stage of broilers' life, but an increase in abdominal fat should be taken into account for carcase quality and processing.