The ESR characteristics are studied for four piperidine nitroxyl radicals in the solidstate and in solutions. They all exhibited singlet ESR spectra in the solid state, but tripletspectra in solutions at a concentration lower than 1×10~(-2)M. The effects of the free radi-eal coneentration and the polarity trod the viscosity of solvents were observed and comparedon five ESR spectral parameters. Linear correlations could be found between the hyperfinesplitting constant A_N and the solvent polarity parameter, namely, Kosower Z value, Ex-perimental data showed also linear relations of the rotational correlation time τ of free rad-icals with the solvent viscosity, η. We have further found that the relative height of thehigh-field peak varied inversely with η. These ESR characteristics of nitroxyl radicals arediscussed. They all extracted singlet ESR spectra in the solid state, but tripletspectra in solutions at a concentration lower than 1 × 10 -2 M. The effects of the free radi-eal coneentration and the polarity trod the viscosity of solvents were observed and comparedon five ESR spectral parameters. The linear correlations could be found between the hyperfine fitting constant A_N and the solvent polarity parameter, namely, Kosower Z value, Ex-perimental data showed We have further found that the relative height of the high-field peak varied inversely with η. These ESR characteristics of nitroxyl radicals are wascluded.