The aim of this study was to evaluate growth performance, selected biochemical blood parameters, and the microbiota of ileal digesta in broiler chickens provided with drinking water containing an addition of natural lavender essential oil (LEO). The experiment was carried out on a commercial farm using n = 300 unsexed Ross-308 broiler chickens. One-day-old chicks were randomly assigned to three groups of 100 chickens each (five replications, 20 individuals each). The control group broilers were provided with drinking water without the addition of LEO. Groups LEO1-42 and LEO22-42 had access to water containing 0.4 ml/L LEO (for 6 h/day) from days 1 to 42 (LEO1-42) and 22 to 42 (LEO22-42). Body weight, feed intake, water intake, and mortality were recorded throughout the experiment. The analyses reveal that the addition of LEO has a positive effect on body weight in the second period of rearing (d 22-24). Treatment broilers (LEO1-42 and LEO22-42) weighed on average 6.35% more compared to the control (P < 0.01). LEO addition positively affected weight gains and feed conversion ratio (P < 0.01) in the second period of rearing (d 22-24). No differences were found between the groups feed intake, water intake, survival rate, and blood biochemical parameters (P > 0.05). The addition of LEO to drinking water had a positive impact on the gut microflora of the ileum: the numbers of pathogenic microorganisms decreased (Escherichia coli and coliform) while the number of probiotic bacteria increased (P < 0.01).