BACKGROUND: The use of additives, such as medicinal plants, may result in improved digestion and fermentation and consequently, animal production. OBJECTIVES: The present experiment was conducted to determine the most appropriate amount of Ziziphora cliniopodiodes plant to be utilized in lamb diet and its effect on digestibility, fermentation, growth performance, blood, and quality of carcass and meat. METHODS: Different amounts of Ziziphora cliniopodiodes (0, 0.2, 0.4, 0.6, 0.8, 1 %) were added to diet of lambs. The best diet was selected based on gas production method and fed to male lambs with an average weight of 41 ± 2 kg in a completely randomized design with three treatments and six replications. The selected diets for feeding lambs included: control diet (Ziziphora cliniopodiodes) and control diet supplemented with 0.2 or 0.4 % Ziziphora cliniopodiodes. Feed intake, digestibility of nutrients, fattening performance, blood and rumen fermentation parameters, protozoan population, and certain meat quality indexes were measured. At the end of the experiment, the carcass traits were studied. RESULTS: The obtained results herein revealed that the potential of gas production, partitioning factor, truly degradable organic matter, dry matter intake, apparent digestibility of dry matter, organic matter, crude protein, neutral detergent fiber, acid detergent fiber, glucose, cholesterol, triglyceride, blood urea nitrogen, and protozoa population were not affected by the experimental diets. There was a significant difference among the experimental diets for daily weight gain, the total weight gain of lambs, and microbial biomass production. The highest records belonged to the diet containing 0.2 % Ziziphora cliniopodiodes. Antioxidant characteristics, colorimetric indices, and pH of meat were not affected by the applied treatments. CONCLUSIONS: In general, according to livestock experiments, the best amount of Ziziphora cliniopodiodes to be employed was 0.2 %, which improved certain fermentative and performance traits.