This experiment was carried out to investigate the effect of adaptation period to oak leave's tannin on rumen pH and ammonia, rumen microbial population, gas production and dry matter true digestibility (IVTDMD) in Alamout Goat’s rumen (with no previous history of oak leaf consumption) compared with Markhoz Goat (with previous history of oak leaf consumption). The goats were fed either oak leaves or Lucerne for 21 d. Dry Matter (DM), Organic Matter (OM), Crude Protein (CP), Ether Extract (EE), Neutral Detergent Fibre (NDF), Acid Detergent Fibre (ADF), lignin (ADL) and phenolic compounds of oak leaves were determined through laboratory analysis. Additionally, ruminal population of cellulytic and proteolytic bacteria and protozoa, pH and the concentration of ammonia-N were found out. An in vitro gas production was employed to estimate organic matter digestibility (OMD), metabolisable energy (ME) and IVTDMD content. Oak leaves’ DM, OM, CP, EE, NDF, ADF, ADL, total phenolic compounds, condensed tannin and hydrolysable tannin, respectively, were 940, 950, 115, 35.0, 520, 320, 98.0, 113, 100, 20.0 and 90.0 g/kg DM. Ruminal pH, ammonia-N concentration, and population of cellulytic and proteolytic bacteria and protozoa of Alamout Goat were, respectively, 6.64, 22.26 mg/dl of ruminal fluid, 7.76, 8.39 and 5.67, for which the adaptation to oak leaves had resulted in a decline in all of them (p<0.01). In vitro gas production volume, OMD, ME, b, c and IVTDMD estimated by use of goat’s ruminal fluid were 19.2 ml, 38.9 g/ kg DM, 5.8 MJ/kg DM, 27.5 ml, 0.05/h and 490 g/kg of DM, which had increased (p<0.01) through an adaptation to oak leaves. On the other hand, addition of PEG to the samples resulted in enhancement of these parameters (p<0.01). In conclusion, adaptation of goats to oak leaves caused a reduction in the negative effect of tannins, and resulted in a change in ruminal fermentation and digestibility.