Wheat straw (Triticum aestivum L.) was treated by using cellulase from Trichoderma viride at 0, 0.2, 0.4, 0.6, 0.8% doses (dry matter (DM) basis) to have straw containing 40% DM. After this process was performed in triplicate for each control and experimental groups, samples ensiled in glass jars were incubated at room temperature (22 degrees C) and 40 degrees C for 30 days. As a result of enzyme treatment, neutral detergent fiber (NDF), hemicellulose and cellulose contents of wheat straws were decreased but water soluble carbohydrate (WSC), lactic acid and metabolizable energy contents (ME) were increased (P < 0.05). While significant increases were determined on in vitro gas production (GP), dry matter digestibility (DMD) and true organic matter digestibility (TOMD) parameters of wheat straws as 23.00 ml, 24.30 and 33.40% (in DM), respectively at 40 degrees C of 0.8% cellulase treatments, microbial biomass production (MBP) was determined as 99.40 mg (in DM) at 40 degrees C of 0.2% cellulase treatment (P=0.000). In conclusion, when the improvements in the nutrient, cell wall component, and in vitro digestibility were considered, it was determined that the use of fibrolytic enzyme increased the nutritive value of wheat straw. The best results were obtained from higher doses of cellulase (0.8%) treatments at 40 degrees C.