The magneto-structural correlation in some p-N-alkylpyridinium nitronyl nitroxide salts has been studied. In the iodide salts of p-N-R-pyridinium nitronyl nitroxides with R=methyl, ethyl, n-propyl, and n-butyl, the magnetic property varies from antiferromagnetic to ferromagnetic with the extension of the N-alkyl chain. The nearest-neighbor molecular arrangements of the nitroxides observed in the four crystals, can be classified into two groups : the radicals are connected by a intermolecular contact between the NO groups (Type I) or between the NO group and the pyridinium ring (Type II). The observed magnetic behaviors can be interpreted in terms of an antiferromagnetic intermolecular interaction for the Type I contact and a ferromagnetic for Type II. Effects of replacement of the I- ion by Br- or Cl-, have been also studied in the p-N-methyl derivative.