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The aim of this study was to investigate the effect of urea treated baled wheat straw on fattening performance of male calves. The bales of wheat straw (about 10 kg each) were treated with urea solution to obtain 45% moisture and 4.5 % urea and were kept under plastic sheet for one month. Then the plastic was removed and samples of treated straw were taken for chemical analysis and in vitro studies. Bio-availability of the straws to rumen microbes was investigated using in vitro gas production and nylon bag techniques. Effect of treated wheat straw on fattening performance of male calves was assessed using 20 heads of Montbeliard male calves in 120 days of fattening period. Animals were divided into equal groups with 10 animals per each. The diet of both groups contained 70% concentrate, 20% wheat straw and 10% alfalfa but in treatment group wheat straw was substituted with urea treated wheat straw. The content of CP in both groups was 14.6% and deficiency of CP in control group was compensated by addition of urea. Animals were fed twice a day and total feed consumption was recorded daily. Animals were weighed every month after 14h of feed and water deprivation. The results showed that urea treatment significantly (p<0.05) increased CP from 2.7% to 6.8% and decreased NDF from 85 to 79 percent. Results of in vitro gas production showed that there was a significant difference (p<0.05) in volume of gas production between the untreated and the treated straws. Urea treatment significantly (p<0.05) increased the fermentation characteristics of b and c and metabolizable energy. Significant increase (p<0.05) was observed in potential rumen degradation and factor of b in the treated sample. Urea treatment did not affect average daily gain and voluntary feed intake but significantly (p<0.05) improved feed conversion ratio (from 8.14 to 7.51).