An unprecedented cyclic Tb-III-radical cluster, [Tb-3(hfac)(6)(4-Me-3-NITtrz)(2)(OH)(3)] (1), has been self-assembled from Tb(hfac)(3)center dot 2H(2)O and 4-Me-3-NITtrz in the presence of Cu(hfac)(2)center dot 2H(2)O. Alternating-current magnetic studies show that the Tb-3 cluster displays a dual slow relaxation of magnetization behavior under a zero direct-current (dc) field and a single relaxation process under an induced dc field with an effective energy barrier of 14 K, representing the first triangle 2p-4f cluster with a slow magnetic relaxation property.