Recently, Iwamura's group has succeeded in synthesizing the interesting new tetranuclear cluster composed of two paramagnetic Mn2+ions (S=5/2 per ion) and two organic free radicals (S=1/2 per radical) as bridging ligand, [{Mn(hfac)2}2(bnn)] (bnn=2,2′-bis[(1-oxyl-3-oxide-4,4,5,5-tetramethyl-imidazolinyl)]). The crystal structure is trielinic (space groupP1) and each cluster is composed of two triangles which share their own base like a “butterfly”. There are two kind of exchange pathways within the cluster and both of them are likely to be antiferromagnetic. Therefore, spin frustration may occur and we expect many possible ground states with different total spins (ST=0 toST=6), depending on the relevant interactions and magnetic field.