This study evaluated the effects of increasing inclusion levels of Gracilaria sp. in complete feed on in vitro rumen fermentation characteristics, including pH, total volatile fatty acid (VFA), and ammonia (NH₃) concentrations. The experiment was conducted using a completely randomized design consisting of four dietary treatments and three replications. The treatments were increasing level of Gracilaria sp. In the diets, i.e. 0% (G0), 5% (G5), 10% (G10), and 15% (G15). Fermentation characteristics were evaluated using the first stage of the Tilley and Terry in vitro technique. Data were analyzed using one-way ANOVA followed by Duncan’s Multiple Range Test. The inclusion of Gracilaria sp. significantly increased (P<0.05) total VFA and NH₃ concentrations but did not affect rumen pH. Total VFA concentration increased from 68.89 mM in G0 to 105.80 mM in G15, while NH₃ concentration increased from 7.82 mM to 10.75 mM. Rumen pH remained within the optimal range for microbial fermentation (6.86–7.00) across all treatments. It can be concluded that the inclusion of Gracilaria sp. up to 15% in complete feed improves rumen fermentation by increasing VFA and NH₃ production while maintaining a stable ruminal pH. The highest fermentation response was obtained at the 15% inclusion level.