The objective of this study was to evaluate the effect of partial replacement of forage with treated straw on performance, chewing activity, rumen fermentation parameters, digestibility, blood parameters and carcass characteristics of fattening lambs. Twenty three (3 to 4 month age and 28 kg weight average out) Afshari growing male lambs were used in a completely randomized design in 104 days period ( 20 d of adaptation followed by 84 d fattening). Forage to concentration ratio was 20:80. Treatments were: 1) control (without treated straw), 2) one third of forages as treated straw, 3) two third of forages as treated straw. At the end of fattening lambs initially weighing the slaughter and hot carcass weight, fat weight and carcass weight offal were measured. The results of this study show Treated Straw increased the proportion of the particles retained on the top sieve of the ( 8 mm) but no effect on third sieve (1.18 mm). DMI lambs fed with control diet, one-third and two-third treated straw respectively 1574, 1538 and 1467 were gr/d experimental diets were not that impressed but it was a significant linear (p 0.05). Average daily gain weight (ADG)during fattening lambs between significantly affected by diets were tested for quadratic (p=0.02), so that the maximum weight of one-third treated straw (330 g/d) and lowest among treatments related to treatment including two-thirds of treated straw (285 gr/d). the feed conversion ratio for second degree tend to be significantly influenced was the replacement level (p= 0.08), so that the lowest conversion rate to treatment was a third straw. While the digestibility of dry matter, organic matter, neutral detergent fiber, acid detergent fiber, crude protein and ash are treated straw in the ration increased linearly with increasing replacement. Eating, ruminating and total chewing time for the control and one-third treated straw was significantly more than two-thirds treated straw treatment. There was no significant difference between rumen fluid PH among treatments and acetate, ammonia nitrogen concentration in treatment 3 was higher than other two treatments.. Total protein and BUN in treatment 2 was higher than other treatments and glucose, cholesterol, triglycerides, aspartate aminotransferase and alanine aminotransferase were not significantly different between treatments. Although, lowest carcass weight and abdominal fat tail and highest weight tail fat in a third treatment was observed, but negative effect of treated straw in treatment 2 was not observed on performance and carcass characteristics. Replace two-thirds of alfalfa hay treated straw with no negative impact on the performance of male lambs Afshari However, the best performance and feed efficiency by replacing one-third straw was treated. Keywords: Treated Straw- Alfalfa hay- Performance- Fattening lambs- Carcass characteristics