Three kinds of diacetylenes of general structure: R1-C≡C-C≡C-R2 (a. R1= R2 = radical; b. R1≠R2 R2 R1 or R2 being radical; c. R1= R2≠ radical) polymerized in the solid state by thermal irradiation method. The magnetic properties and origin of ferromagnetism of these polymers were discussed. The two spin systems-one belonging to the fully conjugated chain and the other to the nitroxyl fragment do nor cause ferromagnetism as they are independent. The ferromagnetism of polydiacetylenes originate from the interaction of the two spin systems.