We have synthesized 1,2-bis[6-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)-2-methylbenzo[b]thiophen-3-yl]hexafluorocyclopentene (2a) which shows photoswitching of intramolecular magnetic interaction between two nitronyl nitroxide radicals. Compound 2 showed ideal photochromic performance. The conversions from open-ring form isomer 2a to closed-ring form isomer 2b, and from 2b to 2a were both almost 100%. From the magnetic measurement it was found that while the two spins of the open-ring form isomer 2a weakly interact with each other (2J/k B =−2.2 K) due to the disconnection of the π-systems, spins of 2b have remarkable antiferromagnetic interaction (2J/k B =−11.6 K). The EPR spectra were measured for both 2a and 2b in benzene solution at room temperature. In both cases, the ESR spectra showed nine lines indicating that the exchange interaction between the two radicals was much stronger than the hyperfine coupling constant. We have also synthesized 3a, in which two nitronyl nitroxides were located at both ends of 1,2-bis(6-phenyl-2-methyl-1-benzothiophen-3-yl)hexafluorocyclopentene. Two p-phenylene groups are introduced to regulate the strength of the exchange interaction between spins at nitronyl nitroxide and to detect the change of the interaction by ESR spectroscopy. Compound 3 underwent reversible photochromic reactions in ethyl acetate by irradiation with 313 and 578 nm light. The photochromic reaction proceeded both ways almost completely. While the ESR spectrum of 3a showed complex 15 lines, the spectrum of 3b showed nine lines. The double integrals of the spectra at the initial and photostationary states were almost identical, indicating that there is no gain or loss of the amount of the spins during the photochromic reaction.