The objective of this study was to evaluate the effect of four essential oils (EOs) from guava (Psidium guajava), citronella (Cymbopogon nardus), lemongrass (Cymbopogon citratus), and geranium (Pelargonium graveolens) on gas production and rumen fermentation in vitro as a natural substitute for the ionophore antibiotic monensin. These EOs are chemically characterized by Gas Chromatography-Mass and evaluated in vitro at four different concentrations (0, 15, 30, and 45 µl per 45 ml buffered rumen fluid) regarding their effects on gas production and rumen fermentation characteristics and were compared to those of monensin. Compared to the negative control, monensin significantly depressed gas production and truly degraded dry matter (TDDM) but enhanced propionate production. All EOs except P. graveolens significantly decreased gas production with increasing concentrations. TDDM was significantly reduced with C. citratus (at 45 µl) and P. graveolens (at 30 and 45 µl). No significant change was detected in the ammonia nitrogen concentration with all assayed EOs except C. nardus and C. Citrus. Compared to monensin and the negative control, C. nardus and C. Citrus reduced the ammonia concentration at high levels. High levels of all tested EOs significantly reduced protozoa counts. The EOs of C. citratus (at 45 µl) and P. graveolens (at 30 and 45 µl) also significantly increased the acetate proportion. Moreover, the acetate to propionate ratio was significantly increasedby30 µl P. graveolens. The results of the current study concluded that the tested EOs, except P. graveolens, efficiently diminished gas production with a similar potency to monensin. Furthermore, they exceed the monensin in their ability to reduce the ammonia nitrogen concentration and protozoa count without adversely affecting volatile fatty acid levels. But, they were less effective than monensin in modifying ruminal volatile fatty acid profile especially propionate and acetate to propionate ratio. Hence, P. guajava, C. nardus, and C. citratus EOs could be a safe and promising rumen manipulator.