BACKGROUND: Essential oils as growth stimulant play an important role in improving intestinal microflora and morphological properties and immune system in broiler chickens. OBJECTIVES: The aim of this study was to investigate the effects of Artemisia sieberi oil on intestinal morphology, secum microflora and immune system in broiler chickens. METHODS: Two hundred day-old broiler chickens (Ross 308) were allocated to 5 treatments, 4 replications with a completely randomized design during 42 days of age. Experimental treatments consisted of control (basal diet) or basal diets containing different levels of Artemisia sieberi oil (100, 200 and 300 mg/kg) and 600 mg/kg flavophospholipol antibiotic. On day 42 days of age, intestinal morphology and microflora population and immune system were evaluated by measuring the weight of bursa of fabricious and spleen and antibody production. Results: The results indicated that the highest lactobacillus count and the lowest Escherichia coli count of the caecum was found by inclusion of 300 mg/kg Artemisia sieberi oil in the diet (p<0.0001). The antibiotic and Artemisia sieberi oil treatments showed higher villus height in the duodenum compared with control group (p<0.01). Artemisia sieberi oil and antibiotic supplementations significantly decreased epithelial thickness and goblet cell number of the small intestinal compared with control group (p<0.05). The dietary supplementation did not significantly affect the crypt depth and villus height to crypt depth ratio in small intestine. The level of 300 mg/kg Artemisia sieberi oil significantly increased antibody titration against Newcastle disease virus (p<0.01). Conclusions: Adding Artemisia sieberi oil at levels of 300 mg/kg to broiler chicken diets can improve gut microflora (as measured by changes in populations of Escherichia coli and lactobacillus).