This experiment was conducted to investigate the effects of folic acid (FA) and coated folic acid (CFA) on lactation performance, rumen fermentation and hepatic FA metabolism in transition dairy cows. Eighty transition dairy cows with similar parity [(2.03±0.18) parities], body weight [(676.0±33.6) kg], milk production in the previous lactation period [(10 155.0±256.2) kg] and expected delivery date were assigned in a randomized block design into 4 groups with 20 cows in each group. Cows in the control group (CON group) were fed a basal diet, while those in the experimental groups were fed the basal diets supplemented with 6.75 g / d CFA (contained 2% FA, CFA group), 135 mg / d FA (LFA group) and 270 mg / d FA (HFA group), respectively. The experiment began from 35 days before calving, and the data and sample collection period was from 0 to 21 days after calving. The results showed as follows: 1) the milk yield, 4% fat-corrected milk yield, milk protein yield and feed efficiency in CFA group were significantly higher than those in CON group, LFA group and HFA group (P<0.05), and the milk fat yield was significantly higher than that in CON group and LFA group (P<0.05). 2) Compared with CON group, the rumen FA content, the activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), and the total volatile fatty acid (TVFA) content in CFA group, LFA group and HFA group were significantly increased (P<0.05), while the rumen malondialdehyde (MDA) content was significantly decreased (P<0.05). Meanwhile, compared with CFA group, the activities of rumen SOD and GSH-Px and the TVFA content in LFA group and HFA group were significantly increased (P<0.05), while the rumen MDA content and the acetate to propionate ratio were significantly decreased (P<0.05). 3) There was no significant difference in the rumen microbiota α diversity indices among the groups (P>0.05). At the phylum level, the rumen Bacteroides relative abundance in LFA group was significantly higher than that in the other groups (P<0.05), while the rumen Firmicutes relative abundance was significantly lower than that in the other groups (P<0.05). At the genus level, the relative abundances of rumen uncultured_rumen_bacterium and Prevotellaceae_UCG-001 in LFA group was significantly higher than those in the other groups (P<0.05), the rumen Rikenellaceae_RC9_gut_group relative abundance in CFA group, LFA group and HFA group was significantly higher than that in CON group (P<0.05), and the rumen Ruminococcaceae_UCG-014 relative abundance in CFA group was significantly higher than that in the other groups (P<0.05). 4) The hepatic FA content and the mRNA relative expression levels of dihydrofo-late reductase (DHFR) and 5,10-methylenetetrahydrofolate reductase (MTHFR) in CFA group were significantly higher than those in the other groups (P<0.05), while the hepatic homocysteine content was significantly lower than that in the other groups (P<0.05). In conclusion, the supplementation of CFA can enhance the lactation performance in dairy cows, which is related to the improvement of rumen fermentation and liver FA metabolism; the supplementation of CFA can stimulate the growth of rumen cellulolytic bacteria, and the supplementation of 135 mg / d FA can stimulate the growth of rumen amylolytic bacteria.