To identification of optimum initial live weight (ILW) and sex effect for fattening of Iranian Lori-Bakhtiari breed, a total of 224 growing lambs were allocated in two separate experiments. The first experiment (n = 144) was conducted to study the optimum initial live weight for more efficient fattening program. The second experiment 80 lambs (40 male and 40 female lambs) was used to investigate the effect of sex of this breed in fattening program in intenstive rearing system. In the first experiment, 144 ram lambs allocated in three different treatments (48 animals per each) with different ILW; T1) 23.2, T2) 27.3 and T3) 32 kg. In the second experiment 80 lambs were assigned in two different treatments (40 male and 40 female lambs as M and F treatments, respectively). Feed intake (FI), average daily gain (ADG) and feed converion ratio (FCR) were determined in both experiments. The first experiment lasted 110 days and the second experiment lasted 100 days. Considering the results for experiment one showed that the best value for FCR was gained when the lambs were fattened at ILW of 23.2 kg and the best ADG also was gained in 27.3 kg ILW. Based on the first experiment, the ILW of about 27 kg could be recommendable for starting the fattening in Iranian Lori-Bakhtiari lambs. Considering the second experiment results show that male lambs had better gain (285 vs. 207 g/d) (P < 0.05) and also lower feed conversion rate (6.31 vs. 6.42) (P < 0.05) in similar age and weight compared to female lambs. This study indicate that male lambs with intitial live weight of 27 kg might be recommendable fattening program in intenstive rearing system.
