The effect of the conformation on the spin multiplicity of the ground state and the stability of the ground state were investigated for m -phenylene type of biradicals by means of comparative study with DFT, CASSCF and AM1-CI approaches. It was found that AM1-CI approach is reliable in dealing with the stability of the high-spin ground state with the change of conformation; DFT method can give the reasonable results of the spin density of the high-spin state. Furthermore, when one or two radical centers are twisted sufficiently out of the conjugation with the benzene ring, m -phenylene turns into weak ferromagnetic and weak antiferromagnetic coupling units, respectively. The effect of the conformation on the spin multiplicity of the ground state and the stability of the ground state were investigated for m-phenylene type of biradicals by means of comparative study with DFT, CASSCF and AM1-CI approaches. It was found that AM1- CI approach is reliable in dealing with the stability of the high-spin ground state with the change of conformation; DFT method can give the reasonable result of the spin density of the high-spin state. flashed out of the conjugation with the benzene ring, m-phenylene turns into weak ferromagnetic and weak antiferromagnetic coupling units, respectively.