The current research focuses on the effect of Conocarpus erectus L. leaves (CL) processed with two tannin-degrading bacteria Klebsiella pneumoniae (K. p) and Acinetobacter (A. b) on digestion activity and fermentation parameters of rumen bacteria and fungi in Arabian sheep. These isolates capable of utilizing tannic acid as sole carbon and energy source. Eight species of Klebsiella pneumoniae (A1, A2, A3, A4, A5, A7, A8, A9) follow by an Acinobacter sp. were used for biological treatment of CL for ten days. Then, digestion activity and some rumen fermentation parameters of ruminal bacteria and fungi in specific culture medium (SCM) were determined in Arabian sheep. Treated CL by bacterial isolates, reduced total tannin (TT) compared with unprocessed groups (P < 0.05). Dry matter disappearance (DMD) was affected by biological treatment for both rumen bacteria and fungi in different incubation times, except for 3 and 6 h of incubation in SCM of mixed rumen fungi, which indicated same pattern with untreated CON group (P > 0.05). The NDF disappearance (NDFD) and CP disappearance (CPD) in SCM of mixed rumen bacteria and fungi were increased during all incubation times (P < 0.05) due to biological prosses of CL compared to CON treatment. Biologically treated CL increased gas production from the fermentable fraction (b) pattern in both SCM of mixed rumen bacteria and fungi during all incubation times (P < 0.05), but, the gas production rate constant (c) was increased only at SCM of mixed rumen fungi (P < 0.05). Overall, the data indicated that bacterial inoculation of K. p and A. b. sp. could improve digestion activity and some fermentation parameters of rumen bacteria and fungi in Arabian sheep.