Aims This study was conducted to evaluate the chemical composition and in vitro gas production (GP) and fermentation parameters of Quercus infectoria and Quercus libani leaves following treatment with the Klebsiella pneumoniae, a tannin-degrading bacterium. Methods and Results This isolate was isolated on medium containing tannic acid as the sole source of carbon and energy, and identified based on 16S rRNA sequencing analysis. In both oak leaf species (i.e. Q. infectoria and Q. libani), inoculation with Klebsiella pneumoniae significantly increased (P < 0 center dot 05) dry matter (DM) loss. For Q. libani, crude protein content was increased (P = 0 center dot 02) by bacterial treatment vs. control. In both oak leaves, total phenolic content and total tannins were decreased (P < 0 center dot 05) as a consequence of bacterial treatment. However, bacterial processing didn't changed (P > 0 center dot 05) organic matter (OM), neutral detergent fibre, acid detergent fibre or acid detergent lignin content of treated leaves. In both oak leaves the measuring parameters including GP volume, in vitro digestibility of DM and OM, estimated metabolizable energy, total volatile fatty acids, acetate, ammonia nitrogen concentration, total protozoal population and the subfamily Isotricha in treatments were higher (P < 0 center dot 05) than control. Conclusions It can be concluded that biological treatment of Q. infectoria and Q. libani leaves with K. pneumoniae represents a useful approach to decrease their phenolic compound content and improve their nutritive value as ruminant feed. Significance and Impact of the Study This study demonstrated that biologically processing of tannin-containing by-products with K. pneumoniae could increase their nutritive value as ruminant feeds and increase animal productivity.