The so called natural microemulsion with O/W type was formulated from plant essential oil (citronella oil), natural surfactant, additive and water via micro-emulsified technique. The citronella oil microemulsion was evaluated for its efficiency of gray mold disease control. Results showed that the microemulsion effectively reduced the disease severity of gray mold in several trials. The efficiency of microemulsion for reducing disease was as same as that of the Topsin-M. The microemulsion was very stable and could maintain its activity for three years when storage under room temperature.