To study the alloying induced drastic antiferro/ferro change of intermolecular magnetic interaction in 1,3,5-triphenyl-6-oxoverdazyl (TOV) radical alloy crystal, (TOV)(1-x)(TOV-H)(x), measurements of high-field magnetization, magnetic susceptibility, and powder X-ray diffraction pattern on this radical alloy have been carried out, where TOV-H is an amine precursor of TOV. The antiferromagnetic (AFM) exchange interaction in pure TOV crystal has changed drastically to ferromagnetic (FM) type by slight alloying (x = 0.03), due to intermolecular distance expansion. The observed magnetization, M(x, H), for the TOV radical alloy at 1.7 K has shown deviation from a simple Brillouin function, and thus the M(x, H) was analyzed approximately by two terms. The composition rate dependence of the lattice parameters of the TOV radical alloy crystals showed an abnormal elongation of the distance of about 15% between radicals along the c-axis at x = 0.03. Based on the experimental facts, we will discuss the effects of elongation of inter-molecular distance on the mechanism of magnetic interaction between the delocalized radical spins.