This study looked at how adding lauric acid (LA) affects methane emissions, fermentation traits, and microbial activity in the rumen fluid of Bach Thao goats, both when they are not stressed by heat and when they are stressed by heat. Methane yield progressively increased in all treatments, with the highest levels observed under HS conditions. Methane emissions were significantly affected by LA or HS from 12 to 72 hours (P < 0.05). LA supplementation significantly reduced methane levels under non-HS and HS conditions, demonstrating its anti-methanogenic properties and potential to mitigate HS-induced methane production. In lab tests, the breakdown of dry matter (DMD) and organic matter (OMD) was much lower during heat stress (HS) compared to normal conditions (P < 0.05). While LA had no significant impact on DMD and OMD under non-HS conditions, it effectively restored these parameters to near-normal levels under HS conditions (P < 0.05). Ammonia concentrations were significantly elevated under HS conditions but were reduced by LA supplementation under both conditions (P < 0.05). However, pH values remained unaffected by LA under both conditions (P > 0.05). Microbial analysis revealed that HS significantly increased the populations of Streptococcus and Methanobrevibacter, key contributors to digestion and methane production, respectively. LA supplementation successfully lowered the numbers of these microbes during HS conditions, helping to keep the microbial balance and making fermentation more efficient (P < 0.05). The findings highlight the potential of LA to reduce methane emissions and maintain rumen fermentation efficiency by modulating key microbial populations.