This study was carried out on 48 weaned male lambs from 3 different indigenous Saudi Arabian sheep breeds (Awassi, Najdi, and Najdi crossbred) in a 3x2 factorial design to evaluate the effect of feeding probiotics (BIO-NUTRA - Direct Fed Microbes, DFM) on growth performance, carcass quality, serum biochemical and hematological parameters, during fattening. Fattening lambs were slaughtered at 6 months of age (45 Kgs average live weight). The obtained results showed that DFM increased weights (P<0.05) of Awassi lambs at 4 months (25.17 vs. 22.67 kgs) and Najdi crossbred at 5 - 6 months of age (32.75 vs. 27.6 & 44.63 vs. 41.4 Kgs) when compared with control ones. Average daily gain of DFM-supplemented Najdi crossbred was subsequently noticed at 4 - 5 and 3 - 6 month periods (0.33 vs. 0.18 & 0.32 vs. 0.28 kg). Differences in body conformation due DFM supplement were significant for body length in Awassi (56 vs. 45 cm) and Najdi (61.8 vs. 49.5 cm), and height in Najdi (74.67 vs. 67 cm) as well as its crossbred (70.5 vs. 65.67 cm) when compared with their control groups. Moreover, the results revealed that probiotics have a positive effect on carcass characteristics. Awassi lambs had the highest dressing % (53.16%), while Najdi control was the lowest (46.52%). On the average, DFM lambs super passed the control ones (P<0.05) in shoulder & forearm weight% (4.18 vs. 3.74 %), Rack weight% (3.59 vs. 3.2%), and tail fat weight % (8.78 vs. 6.11%), but decreased pluck weight% (3.75 vs. 4.18%), leg weight% (6.9 vs. 7.3 %), and meat bone ratio (3.07 vs. 3.57). Genotype by DFM interaction was also evident in Awassi shoulder & forearm weight%, tail fat weight %, fur weight %, and carcass length, as well as Najdi crossbred Pluck weight%, and leg weight %. Evaluation of blood cellular elements and serum biochemical analysis revealed no significant effect due to DFM supplement , except for monocytes and total protein on the whole average (0.55 vs. 0.92 x103/µ& 8.26 vs. 9.36 g/dl), MCH and glucose in Najdi (7.55 vs. 8.5 pg & 69.98 vs. 94.65 mg/dl), MCHC in Awassi (28.3 vs. 26.97 g/dl), and glucose compared to control groups. It would be concluded that DFM may be more economically beneficial for the sheep breeders and the increased meat produced locally can help reduce the need for sheep importing from abroad.