[Background] Loranthus tanakae has rich flavonoids with good pharmacological activities, whereas the studies of the flavonoids are limited because of the shortage of plant resources. [Objective] To isolate and identify the flavonoid-producing endophytes from L. tanakae and study their antioxidant and antibacterial activities in vitro. [Methods] The flavonoid-producing endophytes were isolated from the twigs of L. tanakae and screened by color reaction and thin-layer chromatography (TLC). The total flavonoid content was determined via NaNO₂-Al(NO₃)₃ colorimetry. The biological activities of flavonoid-producing endophytes were preliminarily evaluated by experiments of scavenging free radicals and inhibiting bacteria in vitro. [Results] A total of four flavonoid-producing endophytes, including three fungal strains and one bacterial strain, were screened out and identified as Botryosphaeria sp. (ZC020), Phoma sp. (ZZ105), Nemania sp. (ZS042), and Pseudomonas sp. (ZC026) based on morphological and molecular characteristics. ZC020 and ZS042 had higher total flavonoid content, which was (44.58±0.72) mg/L and (31.98±0.18) mg/L (P<0.05, n=3), respectively. Notably, ZS042 produced the same flavonoids as L. tanakae. ZC026 and ZS042 exhibited excellent antioxidant activity, which showed the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging rates of 72.85%±0.66% and 57.01%±0.89% and the 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) radical scavenging rates of 85.36%±0.75% and 88.17%±0.15%, respectively (P<0.05, n=3). ZZ105 had inhibitory effects on both Gram-positive bacteria and Gram-negative bacteria. [Conclusion] Four flavonoid-producing endophytes were isolated from L. tanakae and one of them was a bacterial strain. This work provides a new resource for the production of flavonoids by L. tanakae and a reference for developing and conserving other rare and precious medicinal plants and materials.