Geranyl diphosphate is the precursor for monoterpenes, the major floral scent in Phalaenopsis bellina. The cDNA of P. bellina GDP synthase (PbGDPS) was cloned, and its sequence contained a glutamate-rich motif (Glu-rich) corresponding to the second aspartate-rich motif (SARM) but not any aspartate-rich motifs (Asp-rich). The recombinant PbGDPS enzyme revealed dual prenyltransferase activities by the production of both GDP and farnesyl diphosphate (FDP). Spatial and temporal expression analyses illustrated that the expression of PbGDPS was flower specific and concomitant with maximal emission of monoterpenes on day 5 post-anthesis. In addition, the expression was strongly related to the intensity of the emitted scent among different plant varieties. This is the first report of a functional GDPS without an Asp-rich motif functioning as a homodimer in a plant (P. bellina, Orchidaceae, monocot).