The aim of this study was to determine the effects of different levels of cellulase enzyme treatments and incubations on digestibility of rice straw. Rice straw was treated by using cellulase from Trichoderma reseei at 0 (Control, C), 1 (RS+ CEL1), 1.5 (RS+ CEL1.5) and 2% (RS+ CEL2) levels (dry matter basis, DM), and treated rice straw samples were ensiled in 6 glass jars for C, RS+ CEL1, RS+ CEL1.5 and RS+ CEL2. For control and each level of cellulase treatment, 3 glass jars were incubated at room temperature (22 degrees C) in the dark for 30 days, 3 glass jars were incubated at 40 +/- 0.2 degrees C in an incubator for 30 days. After the treatment, the significant increases (P< 0.05) were determined in treatment groups compared to control for the percentage of in vitro true digestibility as fed (IVTDas fed), in vitro true dry matter digestibility (IVTDMD), in vitro true neutral detergent fiber digestibility in DM basis (IVTNDFDDM) and in vitro true organic matter digestibility in DM basis (IVTOMDDM) of rice straw. The best results obtained from the highest level of cellulase (RS+ CEL2) treatment at 40 degrees C for IVTDas fed, IVTDMD, IVTNDFDDM and IVTOMDDM were 62.99 +/- 0.22, 60.43 +/- 0.23, 30.70 +/- 0.23 and 63.58 +/- 0.34%, respectively. In conclusion, observed results showed that treatment of rice straw with cellulase improved the true digestibilitiy.