The current study aimed to evaluate the effect of increasing doses of garlic powder (GaP) on in vitro fermentation characteristics. Two successive 24-hour incubations were run, and gas production was measured at the end of each incubation period. Liquid samplings for each dose were reserved to determine ammonia nitrogen (NH3-N) and true organic matter degradability (TOMD). Partitioning factor (PF) was estimated as the ratio between TOMD and the gas produced at 24 hours of incubation. Microbial biomass (MBM) was estimated on the bases of truly degraded substrate and PF. Results showed that gas production increased (P <0.001) with the addition of 32 and 64 mg GaP. An increase (P <0.0001) in NH3-N concentration was recorded with 4 mg and 8 mg GaP compared with control, whereas adding 32 and 64 mg resulted in a NH3-N concentration equivalent to control (averaged 39.25 mg/100 ml). The propionate (C3) increased with doses and the highest proportion was noted with the addition of 8 mg GaP (P <0.001). The TOMD was similar for all the doses except for 64 mg GaP, where a slight but significant (P <0.001) increase was noted (77.7%). GaP did not affect PF and MBM values until the dose of 64 mg. It was concluded that GaP added to a ration composed of 50% roughages and 50% concentrate did not result in drastic modifications of in vitro rumen fermentation parameters, except at the highest dose (64 mg), where an increase of gas production, TODM, PF and MBM were noted.