Twenty four male lambs divided into four groups and were allocated to 90, 105, 120 and 135 days of fattening periods.At the end of each fattening period, lambs were slaughtered. The percentage of cold carcass on the basis of empty body weight in the mentioned periods were 57.0 ± 0.94, 57.5 ±1.7, 59.9 ±3.3 and 62.2 ±4.4,respectively(p<0.05). Moreover, the average of subcutaneous and intramuscular fat percentages were 11.2 ±1.6, 13.4 ±3.2, 15.2 ±2.4 and 16.0 ±2.4,
respectively(p<0.05). Average percentages of crude protein, ether extract, ash and moisture showed significant difference among experimental groups (p<0.01).Moreover, ratio of weight percentage for large and small intestine, blood, abdominal and kidney fat to empty body weight showed significant difference among groups.(p<0.01). Results show that net profit per kilogram live weight gain increases in longer fattening period.Increasing of fattening period cause carcass quality to be reduced. So, the reduction of feed efficiency and carcass quality is not in favor of the national benefits.