A series of imidazolium FeCl4 based magnetic monomers and polymers with different length of alkyl group have been designed and synthesized. The monomers showed paramagnetic property at 300 K when the alkyl group was "short" (n = 2, 4, 6, n represents the carbon atoms number of the alkanes). However, the monomers changed to ferromagnetic when the alkyl group was "long" (n = 12, 18) due to the ordered structure of the alkyl groups in bulk, which will enhance the ferromagnetic coupling between magnetic FeCl4 anions. And, due to the polymer main chain that restricted the freely movement of the alkyl groups, the corresponding magnetic polymers showed lower magnetic susceptibility than the small molecules. By proper design of the monomers and polymers, the magnetism could be tuned, which showed potential applications in design of new magnetic materials.