The objective of the current experiment was to determine the adequate dose and impact of graded concentrations of garlic diallyl disulfide (DADS) and diallyl trisulfide (DATS) on growth performance, total-tract nutrient and energy digestibility, serum immune parameters, and ileal morphology in broiler chickens. At 28-d post-hatch, male broiler chickens were allotted on the basis of initial body weight (1.34 ± 0.106 kg) in a randomized complete block desing (RCBD) to one of six treatments that consisted of an oral gavage of 0, 0.45, 0.90, 1.80, 3.6, or 7.2 mg of DADS + DATS per kg bodyweight (BW) with 8 replicate cages per treatment and 4 birds per cage. The DADS + DATS was administered to birds by daily oral gavage for a period of 6 d. Growth performance was recorded and excreta were collected for analysis of DM, nitrogen (N), and energy (E) digestibility and on the last day of the experiment, the median bird in each cage was euthanized and the mid ileum was excised for morphological and gene expression measurements and blood was collected for serum natural antibody and complement assays. Body weight gain and villus height were linearly increased (P < 0.01) with oral gavage of DADS + DATS. There was a quadratic effect (P < 0.01) of the oral gavage on digestibility of DM, N, and E that corresponded to an average broken-line regression-derived adequate dose of 1.16 mg DADS + DATS per kg BW. Supplementation of DADS + DATS by oral gavage had no impact on gene expression markers although there was a tendency for an increase (P = 0.10) in serum natural antibody activity due to treatment. Results from the current study indicate that supplementation of a gavage containing DADS + DATS improves BW gain, ileal morphology, and digestibility of DM, N, and E and may affect serum immune parameters in broiler chickens. The average broken-line regression-derived adequate dose to optimize BW gain and villus height response was 2.51 mg DADS + DATS per kg BW.