Conformational analyses in the gas phase and the prediction of the crystal structures of 5-carboxy[3.3]-orthoparacyclophane (1) and 6-bromo[4.4]orthoparacyclophane (2) were carried out by using the MM4 force field, and the calculated structures were compared with those determined by X-ray crystallographic experiments. The experimental crystal structure of 5-carboxy[3.3]orthoparacyclophane (1) takes on a parallel conformation where the two benzene rings have a parallel orientation, while the crystal structure of 6-bromo[4.4]orthoparacyclophane (2) is one with quasi-perpendicular conformation where one benzene ring tilts about 60degrees against the other benzene plane. The MM4 conformational analyses in the gas phase based on the calculated Gibbs free energies of the conformers have shown that the MM4 force field reproduces the experimentally determined crystal structures of the two orthoparacyclophanes (I (parallel) and 2 (quasi-perpendicular)) as the dominant conformers. With regard to the crystal structures, the unit cell parameters (a, b, c, alpha, beta, gamma) optimized by using the intermolecular potential energies developed for the MM4 force field agreed very well with experimental ones. The essential factors which govern the stable conformations (parallel or perpendicular) of the orthoparacyclophanes were also examined on the basis of the calculated results by the MM4 (force field) and DFT (molecular orbital) methods.