The objective of this research was to use Bacillus subtilis-based probiotics alone or combined with various additional antibiotic alternatives (yeast-derived prebiotics and zinc supplementation) to maintain gut health and improve growth performance of antibioticree broilers. Three consecutive studies have been conducted to evaluate dietary effects on intestinal morphology, digestive organ development, microbiota, and growth performance of commercial broilers under different coccidia-challenge circumstances. In the first study, broilers were raised under commercial conditions, without coccidia-challenge (Chapter III). Broilers fed diets supplemented with antibiotics exhibited the highest body weight gain (BWG) from d 15 to 27. Broilers fed diets supplemented with B. subtilis or prebiotics + B. subtilis exhibited higher BWG from d 28 to 41 and higher BWG from d 0 to 41 as compared to birds fed control diets. In the second study, broilers were exposed to a clinical coccidia-challenge (Chapters IV and V). Interaction and main effects of the dietary supplement and coccidial vaccination were studied. Diets supplemented with Prebiotics + B. subtilis facilitated broilers to reach a similar feed conversion ratio (FCR) as to the antibiotic control group from d 0 to 56. Feeding diets supplemented with B. subtilis to coccidial vaccinated broilers reduced microbiota diversity by increasing the proportion of an antibiotic-resistant bacterium, Rikenella microfusus. In the third study, interaction and main effects of the subclinical coccidia-challenge and the dietary supplement were studied (Chapters VI and VII). The dietary anticoccidial supplementation increased feed intake and BWG and decreased FCR of broilers from d 15 to 28. However, dietary B. subtilis supplementation did not improve any growth performance of broilers. The combined use of zinc and B. subtilis lowered the high mortality of broilers fed diet supplemented with B. subtilis or zinc alone. Additionally, the dietary supplementation of zinc lowered the proportion of Clostridium in cecal contents of broilers. In conclusion, different nutrition strategies should be considered when broiler chickens are under different health circumstances. Bacillus subtilis-based probiotics have potential to replace the antibiotics, but not anticoccidial feed additives.