Four hundred varieties of plant extracts were screened for inhibitory activity against the NFAT transcription factor which plays an important role in inducing immune response. Among them, the MeOH extract of Cnidium officinale showed potent activity, and the activity-guided separation yielded butylidenephthalide, senkyunolide A and falcarindiol as the active constituents. The $IC_{50}$ value of butylidenephthalide was $1.3{ imes}10^{-4};M$ and was similar to that of senkyunolide A $(2.1{ imes}10^{-4};M)$. Interestingly, falcarindiol showed higher activity $(IC_{50},;2.6{ imes}10^{-5};M)$ than the two phthalides.