These experiments conducted to determine the effect of fibrolytic enzyme (FE) on Bakhtiari and Naieni lambs performances and wheat straw (WS) digestibility ( in-vitro ). In the first experiment, the effect of FE, temperature and pH on in–situ DM disappearance of WS were tested. Experimental treatments were evaluated in a completely randomized design using 3×4×4 factorial experiment as follow: 1- pH level: a- 5.5, b- 5.5 and c- 6.5, 2- FE level: a- control, b- low level of FE ( 0.4 µl/g of WS DM basis), c- medium level of FE ( 0.6 µl/g of WS DM basis) and d- high level of FE ( 0.8 µl/g of WS DM basis) and 3- level of enzyme solution temperature: a- 5 ?C, b- 15?C, c- 25 ?C and d- 35 ?C. Exoglucanase, endoglucanase and xylanase activity were 1446, 1350 and 5091 µmol / min. / ml for Pr-Mote enzyme. Increasing FE concentration increased linearly WS DM disappearance. In second Exp., 20 Nainie lambs were used in complete randomized design with 5 treatments and 4 replicate for 84 days, to investigate the substituting effect of treated WS with FE for alfalfa hay on lamb performances. Pro-Mote enzyme (enzyme activity was same to first experiment) was used for treating WS (0.8 ml/kg WS, DM basis, enzyme activity was similar to the first expriment). Treatment diets were included:1) control (100% alfalfa hay without WS), 2) 10% enzyme treated WS replaced for alfalfa hay, 3) 20% enzyme treated WS replaced for alfalfa hay, 4) 30% enzyme treated WS replaced for alfalfa hay and 5) 30% untreated WS replaced for alfalfa hay. Diet containing 30% WS in comparison to diet containing alfalfa hay decreased dry matter intake, daily gain and body weight and increased feed conversion ratio. But diet containing 30% treated WS compared to diet containing alfalfa hay were not affected daily gain, body weight and feed conversion ratio. In the third experiment, 18 Bakhtiari replacement ewe lambs were used in complete randomized design with 6 treatments and 3 replicate for 84 days, to determine the effect of treating forage portion with two types of FE (Finnfeed Bovizyme and Biovance Pro-Mote), on lamb performance. Exoglucanase, endoglucanase and xylanase activity were 1437, 788 and 7479 µmol / min. / ml for Bovizyme and enzyme activity of Pro-Mote was same to the first experiment. Experimental treatments were included: 1-control (1 ml distilled water/kg forage, DM basis), 2-low level of Bovizyme (1.25 ml / kg forage, DM basis), 3- high level of Pro-Mote (1.75 ml distilled water/kg forage, DM basis), 4- low level of Pro-Mote (0.4 ml / kg forage, DM basis), 3- medium level of Pro-Mote (0.6 ml /kg forage, DM basis), 5-medium level of Pro-Mote (0.6 ml /kg forage, DM basis) and 6- High level of Pro-Mote (0.8 ml /kg forage, DM basis). High level of Bovizyme increased daily feed intake. Comparing to other treatments, high and low level of Pr
