The objective was to explore the possible impacts of dietary supplementation with a synbiotic, organic acids or their combination on the growing efficiency, carcass traits and some blood constituents in broilers. In total, 160 1-day-old male broilers (Ross 308) were randomly assigned into four equal groups (40 birds with 8 replicates). The control group received the basal diet with no supplements, while diets of the treated groups were supplemented with organic acids (OA; 1 g/kg), a synbiotic (S; 1 g/kg) and an equal mix of OA and the synbiotic (OS; 2 g/kg). The dietary supplementation with OA decreased the feed intake (P = 0.011) and bodyweight gain (P = 0.011) when compared with other experimental groups. However, the dietary supplementation with the synbiotic plus OA resulted in a considerable improvement in the overall feed conversion rate when compared with the control and OA-supplemented groups (P = 0.001). Groups fed the diets supplemented with the synbiotic or the synbiotic plus OA showed significantly higher net profits and the profit to cost ratio than did the CON and OA-supplemented groups (P = 0.010 and 0.005 respectively). Birds in the S and OS groups had a significantly higher jejunum villus height (P = 0.001), and carcass (P = 0.012) and breast meat (P = 0.001) yields than did those in the control and OA groups. However, the synbiotic-supplemented group produced significantly lower abdominal fat percentage (P = 0.021). The dietary supplementation with the synbiotic or the synbiotic plus OA decreased the lymphocyte counts (P = 0.003) and the serum reduced glutathione concentration (P = 0.001), but significantly decreased the heterophil to lymphocyte ratio (P = 0.001), and the serum cholesterol (P = 0.014) and oxidised glutathione (P = 0.002) concentrations compared with the control and OA-supplemented groups. The present findings showed that dietary supplementation with a synbiotic or a synbiotic plus OA can improve the growth performance, carcass traits, serum antioxidant activity and blood-related indices in broilers.