The experiment was conducted to evaluate the effects of dietary supplements of either an essential oil mixture (EOM) or a mannan oligosaccharide (MOS) as alternatives to an antibiotic feed additive (avilamycin) for layers kept under hot summer conditions. Four hundred and eighty 54-week old Nick-Brown hens were assigned to four dietary treatments. Each treatment consisted of four replications of 10 cages (three hens per cage). The treatment groups were: 1) Control: Basal diet without additive; 2) Basal diet plus antibiotic (10 mg avilamycin/kg feed); 3) Basal diet plus 1 g MOS/kg feed; 4) Basal diet plus 24 mg EOM/kg feed. Performance of laying hens was affected by dietary treatments. Dietary supplementation of EOM and MOS significantly increased egg production compared with control and antibiotic groups. There were no significant differences in feed consumption between treatments. The EOM significantly improved feed conversion ratio above that of the control group. Egg weights were significantly different between treatments. Laying hens consuming MOS produced significantly lower egg weights than the other groups, while egg weights in the EOM, antibiotic and control groups did not differ significantly. Cracked-broken egg rate was decreased by dietary addition of EOM, MOS and antibiotic compared with the control. Number of deaths among hens was significantly affected by dietary treatments with the number of deaths in the MOS treatment being significantly lower than in the other treatments. The performance of laying hens during the summer season could be maintained with inclusions of EOM and MOS in the diet.