Two feeding trials were conducted to evaluate growth performance, carcass characteristics, nutrient digestibility, haematological and serum biochemical parameters of broiler chickens fed graded levels of onion (Allium cepa) and onion with Bactofort as eubiotics. Two hundred and forty (240) day old Ross 308 strain broiler chicks were allotted into five different dietary treatments with 0, 100, 200, 300 and 400g Allium cepa levels/100kg diet in the first experiment and 0, 15, 30, 45 and 60g Bactofort levels added to 200g Allium cepa/100kg diet in the second experiment. Each treatment had 16 birds per pen replicated three times in a completely randomized design. Feed and water were given ad libitum while vaccination and other daily routine managements were duly observed throughout the experimental period. Data collected were subjected to analysis of variance using Statistical Analysis System and means were compared using Dunnett's package. Results of the first trial showed an insignificant (P>0.05) difference in performance parameters measured during starter period. At the finisher phase, final body weight (2137.47 - 2223.25g) and weight gain (1462.05 - 1548.10g) were significantly (P<0.05) higher in treatments receiving Allium cepa additions compared to the control (0g Allium cepa addition level) which had 2121.53g and 1445.53g respectively while feed intake and feed conversion ratio (FCR) were similar. Mortality was also higher in the control compared to Allium cepa treatment groups. 200g Allium cepa addition level had significantly (P<0.05) higher live weight (2191.78g), dressed weight (1541.11g) and dressing percentage (75.16%) compared to the other treatment groups. However, there were no significant (P>0.05) differences in all haematological and serum biochemical indices across the treatments except for albumin, creatinine, aspartate aminotransferase, alkaline phosphatase and total cholesterol. Percent apparent nutrient digestibilities were also insignificant except for crude fibre and crude protein. Crude fibre digestibility (87.12%) was better in the treatment receiving 200g Allium cepa addition level compared to the control (82.99%). The proximate compositions of meat were significant (P<0.05) except ash across the treatment means. The meat quality characteristics were also insignificant. There was a reduction in ceacal and ileal microbial populations of Allium cepa treated group compared to the control. In the second experiment, the treatment group fed Allium cepa based diet (200g/100kg) with 45g Bactofort addition had better (P<0.05) final weight (2575.00g), weight gain (2529.80g), and FCR (1.89) compared to the rest of the treatment groups. Carcass parameters were all significant (P<0.05) except for percentages of organs weight and dressing percentage. Treatment groups fed 0 and 60g Bactofort addition to the Allium cepa based diet (200g/100kg) had significantly (P<0.05) higher total cholesterol (402.86 and 401.56mg dl-1) and low-density lipoproteins (263.08 and 250.08mg dl-1) compared to the other treatment groups. There were significant (P<0.05) differences in all digestibility parameters. The net profit return was better in the treatment group fed 45g Bactofort addition levels to the Allium cepa based diet (200g/100kg). Moreover, gut microbial populations of coliform, salmonella and E. coli were lower in the treatment group with 45g Bactofort addition level. It can be concluded that 200g/100kg dietary onion powder improved performance, meat quality and modulates gut microbiota in broiler chickens while Bactofort addition at 45g/100kg Allium cepa based diet, similarly enhance performance, nutrient digestibility and cost efficiency in broiler production