Gray mold caused by Botrytis cinerea on Agastache rugosa was occurred at a field of Jinju from 2003 to 2005. The disease symptoms started with water-soaked lesions in the leaves and stem, then these infected lesions became withered, discolored, rotten and died eventually. The fungal pathogen was isolated from the lesions. The fungal conidia were one-celled and mostly ellipsoid or ovoid in shape and light gray in color and 4~20 × 4~13 µm in size. The fungal conidiophores were 14~28 µm in width. The pathogen formed sclerotia abundantly on PDA. The optimum temperature for mycelial growth and sclerotia formation was 20 oC. Pathogenicity of the causal organism was proved according to Koch’s postulates. On the basis of symptom, mycological characteristics and pathogenicity test, the fungus was identified as Botrytis cinerea. This is the first report on gray mold of Agastache rugosa caused by B. cinerea in Korea.