Abstract: The present study was conducted to evaluate the effect of feeding treated wheat straw with whey and urea on fattening lambs performance. About 4.5 kg urea and 1.5 kg salt were dissolved in 40 L water and mixed with 30 L of whey and final solution spreaded with over 100 kg chopped wheat straw. Thirty two male lambs were randomly divided into four feeding groups. Each treatment consisted 8 male lambs during 85 days experimental period to compare performance with diets based on barley supplemented with 30% wheat straw. Treatment 1 (control) were fed 30% Untreated Wheat Straw (UWS), treatment 2) 20% UWS+10% Treated Wheat Straw (TWS), treatment 3) 10% UWS+20% TWS, treatment 4) 30% TWS. The animals were slaughtered after 85 days of feeding and carcass and some meat quality parameters were measured. Average final weights of lambs did not differ significantly between experimental groups. However, average live daily and weight gain were significantly (p<0.03 and p<0.001, respectively) higher in lambs fed treatments 2 and 3. Feed conversion ratio was lowest in lambs fed 20% TWS (8.34) and 10% TWS followed by those fed control diet (10.95) which in turn were superior to those fed the 30% TWS diet (p<0.03). No differences were observed for live weight gain between parameters and carcass weights were similar for the diets. Dissection of different anatomical parts showed a higher percentage of lean meat, carcass fat and internal fat in animals fed treated samples compared to control ones. The results of this experiment indicate that treated wheat straw silage can replace as part of untreated wheat straw. The lambs, indeed maintained similar growth rates compared to those given the control (traditional) diet and the feeding cost per kg of body weight gain was lower in the experimental diet. Carcass and meat quality were not affected by treatments and thus, the diet containing treated wheat straw could not represent an economic advantage for procedures. At the end of the trial, the findings were not statistically important (p<0.05). In conclusion supplementation of treated wheat straw with a barley based concentrate support had not positive effect on fattening lambs performance before and after slaughtering.