Twenty four crossbred male lambs (Finn-Ossimi), were randomly assigned to four nutritional groups, to receive one of the following experimental rations for 77 days. Treatments were; T1: 70% CFM + 30% BS, T2: 70% CFM + 30% BS + 3% fumaric acid (FA)/roughage ratio, T3: 70% CFM + 30% (BH) and T4: 70% CFM + 30% BH + 3% fumaric acid (FA)/roughage ratio. Experimental rations used, based on concentrate pelleted feed mixture (CFM, 15% CP) and either bean straw (BS) or berseem hay (BH) as a daily roughage sources. The main objective of the study was to evaluate the effect of fumaric acid supplementation to growing male lambs rations on ruminal fermentation activity and lambs growth performance. Animals live body weight gain, feed intake (g/h/d) and feed conversion ratio (kg intake/kg gain) were estimated. A metabolic trial was conducted, to evaluate experimental rations digestibility, NV, NB and ruminal fermentation measurements. Results obtained indicated significant differences (p < 0.05) in rations nutrients digestibility values. Both the two control rations within each roughage source indicated lower digestibility values in compare with the corresponding supplemented ones. Higher (p < 0.05) nutritive values were recorded by T3 and T4 (BH-F groups) in different feeding terms, i.e. higher TDN, DCP and C/P ratio, in compare with the corresponding BS rations. Different experimental groups indicated significant NI/h/d, while both the two dietary supplemented groups recorded higher NI in compare with their corresponding control ones, however, (ND and NB), indicated insignificant differences among different experimental groups. The effect of dietary fumaric acid supplement on some ruminal measurements indicated significant differences (p < 0.05) among different experimental groups in favor of both the two supplemented groups. Growth performance of crossbred male lambs, indicated insignificant higher daily gain for both the two BH groups in compare with the corresponding BS groups, besides higher insignificant faster daily gain values for both the two supplemented groups in compare with the corresponding control ones. FC ratio favored both the two BH groups as the more efficient feed utilizers in compare with both the corresponding two BS groups, while both the two supplemented groups surpassed insignificantly both the two control groups, within each roughage source. Results of economic efficiency indicated significant differences among different experimental groups in feed cost/kg gain, Both the two control groups recorded lower (p < 0.05) feed costs/kg gain, while supplementing the control rations with fumaric acid led to increase (p < 0.05) daily feed costs/kg gain and reduce (p < 0.05) the net profit value. On the light of the present results it could be concluded that supplementing the control rations T1 and T2 with fumaric acid did not lead to an obvious advantages on animal performance.