Magnetic microcapsules with inner asymmetric structure are prepared by miniemulsion polymerization of styrene in presence of oleic acid-modified Fe3O4 as magnetic nanoparticles and n-heptane (HP) as core. The resultant microcapsules with novel structure exhibit magnetic anisotropy and cavity anisotropy. The formation mechanism of the microcapsule is illustrated in detail. The obtained results showed that the polymerization process is associated with phase-separation between polystyrene, oleic acid (OA) and HP. Based on this formation mechanism, a kind of highly asymmetric magnetic microcapsules are prepared, which is further verified the presented phase-separation mechanism. Meanwhile, four kinds of magnetic microcapsules with inner asymmetric structure, which show different shell thickness and cavity volume, are prepared for application to carrying and releasing drugs. The results reveal that the microcapsules exhibit excellent controlled drug release property and the release speed can be controlled by adjusting the thickness of the thinner-side capsule-wall. Drug loading and drug release of inner asymmetric magnetic microcapsules were from the thinner side of capsule-wall. This kind of novel microcapsule shows potential application value in unilateral or contact point controlled and concentrated release. (C) 2015 Elsevier B.V. All rights reserved.