Poly(phenylenevinylene)s with built-in radical groups are expected to show magnetic interaction caused by spin polarization and/or coplanarity of the pi-conjugated chain. Diradical models for the poly(phenylenevinylene)-based polyradicals were synthesized, and the magnetic properties were discussed based on ESR spectra and magnetic measurement. Magnetization and magnetic susceptibilities of the polyradicals, tert-butylnitroxyl- and 2,6-di-tert-butylphenoxyl-substituted poly(1,2- or 1,4-phenylenevinylene), indicated the occurrence of ferromagnetic interaction corresponding to these oligomeric diradicals.