Abstract: Background and Objectives: Enhancing poor quality roughages by biological treatments has interested for many researches in last years. So, in the current study was carried out to production of tannase enzyme by Aspergillus terreus and its impact on ruminal fermentation, degradability and gas production were evaluated through in vitro trials. Materials and Methods: Aspergillus terreus was grown as stand cultures in 100 mL conical flasks containing tannic acid powder medium. The maximum production of tannase by Aspergillus terreus was achieved at inoculum ratio of 4% (v/v), 72 h of incubation period, initial pH 5.0, urea as a nitrogen sources at a concentration of 0.33 g N L–1 and pomegranate peel as a carbon source at a concentration of 10% (w/v). For animal feeding experiments in vitro dry matter, NDF, ADF, cellulose and hemicellulose disappearance and rumen fermentation were determined for the experimental diets. The concentrate:roughages ratio was 50:50 on DM basis, experimental diets plus different levels of tannase enzyme being 0, 5250, 10500, 15750, 21000 and 26250 IU kg–1 DM were studied. Results: Tannase enzyme increased DM, NDF, ADF, cellulose and hemicellulose degradability of the treated diets compared with the control diet (0.0 IU kg–1 DM), which gave the lowest values of diet degradability parameters. The maximum produced tannase efficiency percentage for dry matter degradability was obtained at 15750 IU kg–1 DM. Production of tannase enzyme by Aspergillus terreus strain under solid state fermentation was superior over tannase production from other fungal strains under the submerged cultures. Conclusion: In conclusion tannase enzyme had positive improving of feed digestion in the current in vitro study.