The objective of this study was to determine the effect of Ltyrosine oral administration (100 mg/kg B.W.) on growing lambs performance from weaning till puberty. Growth performance till sexual activity, testes circumference, urethral process length, semen characteristics, testosterone and thyroid hormones levels were recorded. Thirty-two ram lambs and ewe- lambs were randomly divided into two equal groups, the first group (n=8♂ & 8♀) was treated by L-tyrosine, and the second group (n=8♂ & 8♀) was untreated and served as control. These animals were born in the lambing season of autumn (September/October 2007). Ewe-and ram-lambs were weighed monthly. The ewe-lambs were kept with the ram lambs during the experimental period. Blood samples were collected at 0, 6, 8 and 10 months post-weaning from ram-lambs to determine levels of testosterone, triiodothyronine (T3) and thyroxine (T4) hormones concentrations in plasma. Also, blood samples were collected at the same time from ewe-lambs to determine levels of T3 and T4. The results revealed that lambs treated with L-tyrosine attained puberty at younger ages and heavier body weights compared to the untreated ones. Total gain and daily gain in both ram-and ewe lambs were increased significantly in treated lambs compared to the control. Triiodothyronine (T3) concentration was increased significantly (P<0.05), while, thyroxine (T4) concentration was increased insignificantly in blood plasma by administration of Ltyrosine. Also, thyroid hormones concentrations were increased significantly (P<0.01) by progressing in age until they reached the highest concentrations at ten months old. Moreover, testes circumference and urethral process length were insignificantly increased, but, libido was improved significantly (P<0.01) in treated ram- lambs compared to the control group. Testosterone hormone concentration was increased significantly (P<0.05) in treated ramlambs compared with the control group. Most of the physical semen properties were improved significantly (P<0.01) in treated ramlambs. Finally, L-tyrosine oral treatment was accompanied with better economic returns.