Novel smart thermoplastic magnetorheological elastomer composites containing micron-sized magnetic carbonyl iron (CI) particles were prepared with a poly(styrene-ethylene-butylene-styrene) (SEBS) triblock copolymer utilized as the thermoplastic matrix rubber, and the structures and properties of the CI-SEBS composites were examined. The CI particles were uniformly dispersed in the composites prepared in the absence of the magnetic field at high temperatures T (>T-g(S)), and this isotropic composite exhibited a larger storage modulus G' compared to the SEBS matrix at room temperature (<< T-g(S)) where the EB phase therein was rubbery while the PS phase was in the glassy state. In contrast, the SEBS composite prepared under the magnetic field (with the intensity psi < 2.5 T) at high T (>T-g(S)) contained a chain structure of CI particles. This chain structure became longer and better aligned on an increase of psi up to a saturation of the particle magnetization and on an increase of the time interval of applying the field (that allowed the particles to move and equilibrate their aligned structure). The modulus G' of this "pre-structured" composite measured for both cases of psi = 0 and psi > 0 in the direction perpendicular to the chain structure at room temperature was enhanced compared to G' of the isotropic composites. This difference of the filler effect (for psi = 0) and the magnetorheological effect (for psi > 0) between the pre-structured and isotropic composites was enhanced when the chain structure of the CI particles in the pre-structured composites became longer and better aligned. A mechanism(s) of this enhancement was discussed in relation to the morphologies (particle distribution) in the composites with the aid of a filler model and a molecular expression of the stress due to magnetically interacting particles.