The inevitable deficiency in nutrients and energy at the onset of lactation requires an optimal adaptation of the hepatic lipid metabolism to overcome metabolic stress. Fatty liver is one of the main health disorders after parturition. The regulation of hepatic lipid metabolism is largely dependent on mitochondria. The aim of this study was to investigate the changes in hepatic mitochondrial function in dairy cows with mild fatty liver. Liver and blood samples were collected from healthy (n = 15) and mild fatty liver (n = 15) cows. The mRNA levels of oxidative phosphorylation complexes (CO I-V) and the protein levels of the mitochondrial function regulators were detected by RT-qPCR and Western blotting, respectively, and analyzed with paired t tests. The number and volume of mitochondria in the liver were detected by transmission electron microscopy. The hepatic ATP content and mitochondrial DNA (mtDNA) were measured using biochemical kits and analyzed with Wilcoxon-signed-rank-test. Healthy cows were defined as less than 1% liver TG on wet weight basis and mild fatty liver cows were defined as 1–5% liver TG on wet weight basis. Compared with healthy cows, ultrastructural analysis revealed a significantly increased number and volume of mitochondria in the liver of mild fatty liver cows. Hepatic mtDNA was also markedly increased in mild fatty liver cows. These results indicate that mitochondrial function is enhanced in the liver of mild fatty liver cows. We further explored the molecular mechanisms involved. The protein levels of mitochondrial function regulators, PGC-1α and NRF1, were significantly increased in the liver of mild fatty liver cows. The mRNA levels of CO I-V regulated by PGC-1α were significantly higher in the liver of mild fatty liver cows than healthy cows. The ATP content was increased, which was consistent with the increase of CO I-V. In summary, our data show that the hepatic mitochondrial function of dairy cows with mild fatty liver is enhanced, and enhanced mitochondrial function may be the adaptive mechanism of dairy cows with mild fatty liver.