In order to develop ultra-thin magnetic films, we investigated the structure and magnetism of Langmuir-Blodgett (LB) multilayers of polydiacetylene by X-ray diffraction and electron paramagnetic resonance (EPR). Using X-ray diffraction, we determined the orders of interlayer and of in-plane directions of the LB films. Upon ultraviolet irradiation, the LB film was found to tilt and to increase the ordering. In EPR measurements, angular variation of linewidth of monomer PDA LB films indicated a spin chain interaction of one-dimension. On the other hand, the polymeric PDA LB films exhibited a drastic extinction of EPR signal due to the onset of antiferromagnetic coupling by the decrease of distance between Mn ions. We concluded that the PDA LB films may permit the novel order structure with the low-dimensional spin magnetism.