The aim of the present study was to examine the effects of a herbal essential oil mixture on the performance of broilers produced by a young (30 wk) or an old breeder (80 wk) flock. One thousand and eight unsexed day-old broiler chicks (Ross-308) originating from the two breeder flocks were randomly allocated to three treatment groups of 336 birds each. Dietary treatments were: a control and two diets containing 24 mg/kg or 48 mg/kg of an essential oil mixture. There were no significant effects of dietary treatment on body weight of the broilers at 21 and 42 days. The effect of the age of the parents did not have a significant effect on body weight of the broilers at 21 and 42 days of age. Up to 21 days the feed intake of broilers from young breeders was reduced significantly as a result of the inclusion of the essential oil mixture in their diets, and a significant improvement in feed conversion ratio was recorded in these birds. Difference in regression coefficients for feed intake and feed conversion ratio between broilers from young and old breeder flocks was significant. Carcass yield and some internal organ weights such as the liver, pancreas, proventriculus, gizzard and small intestine were not affected by the addition of the essential oil mixture to the diet. Inclusion of essential oil mixture to the diet decreased mortality significantly at 21 days.