Farnesyl diphosphate (FPP) synthase (FPS) catalyzes the condensation of isopentenyl diphosphate (IPP) with allylic primer to give FPP as final product. The FPS from pig liver has been successfully applied to syntheses of bioactive compounds. Molecular cloning and expression of the gene for thermostable FPS from Bacillus stearothermophilus made it possible to produce sufficient amounts of the enzyme. However, it can hardly accept the substrate analogs having oxygen atom in their alkyl chain, which are easily accepted by pig liver enzyme. We constructed many mutated FPP synthases (FPSs) from B. stearothermophilus, in which Tyr-81 was substitute with Ser (S), Arg (R), Asp (D), or Gly (G). Interestingly, the substrate specificities of the mutated enzymes have been dramatically altered. They can easily accept the substrate analogs having oxygen atom. These results indicate further possibilities in application of the mutated enzymes to organic synthesis.