The aim of the current experiment was to determine the effect of inclusion of oak acorn on the gas production, methane production, digestibility, partitioning factor, microbial protein production, and efficiency of microbial protein protein production of lamb diets. In this project, oak acorn was included into lamb diets at the ratio of 0, 10, 20 and 30%. The inclusion of oak acorn decreased the gas production, methane production, and digestibility whereas inclusion of oak acorn increased partitioning factor, microbial protein and efficiency of microbial protein production. Gas and methane production ranged from 89.93 to 129.22 ml and 10.43 to 19.65 ml. Percentage of methane production ranged from %11.53 to 15.22. Digestibility ranged from 61.63 to 72.90%. Partitioning factor and microbial protein production ranged from 3.38 to 4.10 and 128.57 to 143.91 m respectively. Efficiency of microbial protein production ranged from 34.80 to 46.31%. Although oak acorn had an anti-methanogenic potential, oak acorn decreased the digestibility. It seems to be very difficult to determine the proper inclusion dose of oak acorn. Therefore in vivo digestibility studies are required to determine the proper inclusion dose of oak acorn to lamb diets.