This study was conducted to investigate the effects of the medicinal plant yarrow (Achillea millefolium L.), a probiotic (Primalac) and an antibiotic (virginiamycin) on gastrointestinal tract (GIT) characteristics, microbial populations, immune response, serum lipids and growth performance of broiler chickens. A total of 250 one-day old male broilers (Ross 308) were randomly allocated to 5 treatments, 5 replicates with 10 birds in each in a completely randomized design. Experimental treatments included the control, yarrow powder at two different concentrations (1.5 and 3% of diet), Primalac (0.1% of diet) and virginiamycin (15 ppm). The highest feed conversion ratio (FCR) was observed in the control while the lowest FCR was seen in the virginiamycin group at 42 days of age (P< 0.05). Moreover, the highest body weight gain (BWG) was observed in the virginiamycin group while the lowest value was related to the control animals (P< 0.05). Carcass yields were not different between treatments (P> 0.05). Relative weights of breast and thigh were similar among all treatments (P> 0.05). Relative weights of bursa Fabricius, spleen and primary immune response (total titer, IgG and IgM) against sheep red blood cells (SRBC) were not affected by treatments. The serum cholesterol, triglyceride as well as high and low density lipoprotein (LDL and HDL) levels were different among treatments (P< 0.05). The lowest concentrations of the mentioned parameters were obtained in the group supplemented with 3% of yarrow (P< 0.05). The highest and lowest antibody titers (secondary immune response) against SRBC were observed by the yarrow (3%) and antibiotic supplementations, respectively (P< 0.05). The highest lactic acid bacteria (LAB) counts were detected in the crop, ileum and cecum of the Primalac group (P< 0.05). Inclusion of virginiamycin and yarrow (3%) caused a significant decrease in coliforms and total aerobic bacteria counts in crop, ileum and cecum (P< 0.05). The results of this study showed that the administration of yarrow (3%) can reduce the levels of serum lipids and boost the immune response in broilers. Moreover, it led to reduced pathogenic bacteria population in the GIT which could help to improve intestinal health and well being of poultry. It is proposed that yarrow can be used as an antibiotic alternative.