Self-assembly induced increase of magnetic susceptibility for the tadpole-shaped block copolymer based on paramagnetic FeCl4 pyridinium was reported in this work. The tadpole-shaped block copolymer was prepared by sequential reversible addition-fragmentation chain transfer polymerization of styrene (St) and 4-vinylpyridine (4VP) at the presence of chain transfer agent-modified polyhedral oligomeric sil-sesquioxanes (POSS). The corresponding magnetic block copolymer (POSS-PSt-b-QP4VP(FeCl4)) was obtained by quternization of 4VP and complexation with FeCl3. The prepared block copolymer was paramagnetic with magnetic susceptibility of 15.4 x 10(-6) emu g(-1), as investigated by superconducting quantum interference device. Surprisingly, the magnetic susceptibility of the block copolymer was almost doubled (27.5 x 10(-6) emu g(-1)) after self-assembly in thin film. Since the composition of the block copolymer is unchanged, the increase of the susceptibility should be generated by the self-assembly, which makes the magnetic FeCl4 more ordered. And, self-assembly will be an efficient and convenient method in improving magnetic properties of such polymers. (C) 2017 Elsevier Ltd. All rights reserved.