The medicinal fungus Ganoderma lucidum contains many bioactive triterpenoids, ganoderic acid A (GAA) being one of the major ones. The present study explored the microbial biotransformation of GAA, isolating 283 strains of soil actinomycetes and determining their abilities to biotransform GAA with ultra-performance liquid chromatography analysis. One positive strain, AI 045, was selected to validate the biotransformation activity. The strain was identified as Streptomyces sp. based on the sequenced 16S rRNA gene. The produced compound obtained from the biotransformation of GAA was purified with the preparative high-performance liquid chromatography method and identified as 3-O-acetyl GAA based on mass and nuclear magnetic resonance spectral data. The present study is the first report that bacteria have the novel ability to biotransform the triterpenoids of fungus G. lucidum. Moreover, the identified 3-O-acetyl GAA is a new triterpenoid product discovered in microbes.