Well-defined magnetic polyelectrolytes with tetrachlorideferrate (FeCl4-) as counter ion are prepared. In this approach, norbornene-based monomer containing ammonium chloride group (TAENDI-Cl) is designed and synthesized. Well-defined magnetic polymers (Poly(TAENDI-FeCl4)) are obtained by ring-opening metathesis polymerization of TAENDI-Cl in the presence of Grubbs third generation catalyst followed by complexing with FeCl3. Magnetic block copolymers are thus prepared. Both the monomer and polymers are paramagnetic as measured by superconducting quantum interference device method. Studies show that the magnetic susceptibility increases with increasing degree of polymerization (DP) and reaches maximum at DP of 100, and then decreases with increasing DP. Block copolymer with lower FeCl4-content shows higher magnetic susceptibility. And, by introducing FeCl4-, the polymers show obviously magnetic responsive in solution, powder, and film which have potential applications in magnetic switching, transport, and separation.