In order to study the influence of a 6′-methyl group in ring C of griseofulvin (1＿) on the fungicidal activity, 6′-methyl group was replaced with a larger phenyl group as (±)-6′-phenylgriseofulvin (3＿), (±)-6′-epiphenylgriseofulvin (4＿), synthesized by a Diels-Alder cycloaddition. Their biological activities were examined against Botrytis allii (IFO 9430) and B. cinerea (AHU 9573). (±)-6′-Phenylgriseofulvin (3＿) showed high activity in 25 ㎍/disc.