The present study was conducted to investigate effect of the fattening system and the duration of the fattening period on growth performance and carcass composition. in lori Bakhtiary Male Lambs. Sixty lambs were assigned to a factorial experiment of a completely randomized design comprised of two factors, including: fattening system (FS1: feedlot following weaning, and FS2: grazing on pasture followed by feedlot) vs. the length of the fattening period (75, 90 and 105 days). Lambs were weaned at 90 ± 5 days of age. The two fattening system lambs were randomly divided into three groups of 75, 90 and 105 days on feedlot and were kept in individual boxes. At the end of fattening period, half of the lambs of each level of fattening period were randomly slaughtered. The results indicated that the average final weight (61.23 vs. 62.34 kg), daily gain (198.3 vs. 209.2 gr) and feed conversion ratios (8.50 vs. 9.31) were not significantly (P < 0.05) different between FS1 and FS2 lambs. This was while the feed intake in FS2 lambs was significantly (P < 0.05) higher than that in FS1 lambs.