One hundred and seventy four Ghezel and 130 Mehraban weaned male lambs were utilized to study the effect of on post weaning growth on slaughter weight and carcass traits. The lambs in each breed were randomly assigned to predetermined slaughter weight groups of 30, 40 or 50 Kg and were so far fattened as to reach their assigned slaughter weights. The average post weaning daily gain was estimated to be 185.82 (±54.88) grams, increasing significantly (p<0.05) with as increase in slaughter weight. Ghezel male lambs showed a significantly (p<0.05) more average daily gain, needing less time to reach their assigned slaughter weights than Mehraban ones. The dressing out percentage, hot carcass, cold carcass, carcass cuts' weights as well as cuts' percentages increased significantly (p<0.05) with increasing slaughter weight in both the examined breeds. The mean of dressing out was estimated to be 48.06 (±3.3) and varied between 45.78 (±0.31) and 51.36 (±0.41). All non-carcass fats and back fat thicknesses increased (p<0.05) significantly with increase in slaughter weight. The mean back fat depth varied from 2.56 (±0.217) to 6.64 (±0.286) mm for Ghezel slaughtered at a weight of 30 Kg and Mehraban slaughtered at weight of 50 Kg, respectively. Based on the obtained results, it can be concluded that the lambs of Ghezel can be fattened for more than 50 Kg, but slaughter weight of 50 Kg seems to be a proper target weight for Mehraban lambs at which to be slaughtered.