This study was conducted to assess the in vitro organic matter digestibility (OMD) of pistachio hull using rumen fluid of Taleshi sheep. Animals were fed diets composed of 34% pistachio hull, 29% grape pomace, 32% alfalfa, and 5% grain barley. The chemical composition and phenolics' levels of pistachio hull, either Industrially Dried Pistachio Hull (IDPH) or sun Dried Pistachio Hull (SDPH) were determined. In vitro gas production for 24 h either with or without Polyethylene Glycol (PEG) addition was made use of estimate the OMD for IDPH or SDPH for either of the days 0, 3 or 30. For IDPH, the OMD on days 0, 3 or 30 (with no PEG) were respectively estimated as 54, 65.6 and 63.4, while with PEG application they were 65.2, 71.3 and 70.1 respectively (P<0.05). Also, the OMD for SDPH on days 0, 3 or 30 (with no PEG) were estimated as 52.6, 62.9 and 60.3 respectively and, which with an addition of PEG reached 62.1, 68.6 and 67.3 respectively (P<0.05). The results finally revealed that the microbial activity in the ruminal ecosystem of Taleshi sheep have the capability to degrade the hydrolysable tannin in pistachio hull to be later used as a source of energy.