Ganoderma mushrooms are widely used in clinical therapies and functional foods. The antidiabetic effect of Ganoderma has become a research hot spot in recent decades. To search for a superior antidiabetic Ganoderma extract, five common Ganoderma species (G. lucidum, G. sinense, G. tsugae, G. applanatum, and G. leucocontextum) were investigated. A total of 10 fractions, including a total triterpenes fraction and a crude polysaccharides fraction for each, were prepared for further assays. Activities of α-glucosidase and α-amylase are inhibited dominantly by triterpenes from all five Ganoderma species rather than the polysaccharides. G. lucidum triterpenes inhibits α-glucosidase and α-amylase most significantly with IC50 values of 10.02 ± 0.95 µg/mL and 31.82 ± 4.30 µg/mL. Even more, triterpenes content was positively correlated with anti-α-glucosidase and anti-α-amylase activities. Therefore, triterpenes were considered to be the active compounds in inhibiting α-glucosidase and α-amylase activity. It is hoped that the results will provide more systematic information for the application of Ganoderma in the functional food and traditional medicine industries in the future.