Magnetorheological (MR) elastomers are a group of smart materials whose modulus can be controlled by the application of an external magnetic field. In this paper, MR elastomers based on isobutylene-isoprene rubber were prepared by the common manufacturing procedure of rubber and the corresponding MR effect, mechanical properties, and thermal stability were investigated. The results showed that MR effect varied with the volume content of iron particles and a maximum of 20% in MR effect was obtained at 15 vol% of iron particles. The relationship between MR effect and microstructure was discussed in detail. Mechanical tests showed that iron particles could improve the tensile strength and hardness. However, compared with carbon black with the same volume content, the reinforcing effect was far worse. TG analysis showed the thermal stability of isobutylene-isoprene rubber was improved by incorporation of iron particles.