Magnetic susceptibilities of 8 kinds of 3-(4-R-phenyl)-1,5-diphenyl-6-oxo-and-thioxoverdazyl radical crystals (p-MDpOV (R = OCH3), p-MeDpOV (R = CH3), p-CyDpOV (R = CN), p-NDpOV (R = NO2), p-MDpTV (R = OCH3), p-MeDpTV (R = CH3), p-CyDpTV (R = CN), and p-NDpTV (R = NO2)) were measured between 4.2 and 300 K. The susceptibilities of p-MDpOV, p-CyDpOV, and p-MDpTV can be well explained by a one-dimensional(1D) antiferromagnetic (AFM) Heisenberg linear chain model with exchange interaction of 2 J/k = -27.3, -27.3, and -10.9 K, respectively. The susceptibility of p-MeDpTV can be interpreted in terms of a 1D AFM Heisenberg alternating chain model with 2 J(1)/k = -92.0 K (alternation parameter a J(2)/J(1) = 0.7) On the other hand, the susceptibilities of p-MeDpOV, p-NDpOV, and p-CyDpTV follow the Curie-Weiss law with positive Weiss constants of +2.5, +1.5, and +2.9 K, respectively, and the exchange interactions 2 J/k were estimated to be +7.0, +5.0, and +7.0 K, assuming the one-dimensionality for these radical crystals. The susceptibility of p-NDpTV follows the Curie-Weiss law with a negative Weiss constant of -0.9 K. Ab initio molecular orbital (MO) calculation was performed for five kinds of 6-oxoverdazyl radicals to clear the origin of the intermolecular ferromagnetic exchange interaction that is observed with high probability for these verdazyl radicals. A very strong spin polarization effect has been found, which is advantageous to the intermolecular ferromagnetic interaction.