the objective of this study was to evaluate the effect of partial replacement of soybean meal (SBM) as traditional source of protein with optigen as slow-release urea in rations of growing lambs on growth performance, ruminal fermentation, digestibility and economical efficiency. Twenty four Rahmani lambs with average initial live body weight (LBW) of 25.97±0.07 kg were divided into three similar groups (8 in each). All lambs were fed rations composed 2% of their LBW concentrate feed mixture (CFM) plus 1% of LBW clover hay (CH). Lambs in the control group were fed the control ration without optigen (R1), while in the other groups, optigen was supplemented at levels of 0.9 and 1.8% of CFM instead of 15.81 and 31.07% of the protein content of CFM used for R2 and R3, respectively. Chemical composition of tested rations were nearly similar in their contents of all nutrients. The highest (P<0.05) digestibility of all nutrients and nutritive values were recorded for R2, while control ration (R1) showed the lowest values and R3 achieved intermediated values. No significant differences were observed among tested rations and sampling times for ruminal pH values, expect R2 recorded significant difference between sampling time. While TVFA's and NH3-N concentrations were significantly (P<0.05) differ among the different rations and sampling times. The DMI was nearly similar for different groups, while TDN, DCP and DE intake were significantly higher (P<0.05) in 2nd group compared with control,whereas 3rd group recorded intermediate values. Final body weight, total and daily weight gain were significantly higher (P<0.05) with R2 compared to R1 and R3. However, feed conversion ratio did not significantly difference for the different rations. Rations contained different levels of optigen achieved significantly (P<0.05) higher economic efficiency than control ration. It could be concluded that the inclusion of optigen at level of 0.9% in CFM in the diet of growing lambs were more economical efficiency and can be used to replace a part of soybean meal without any adverse effect on their performance