Magnetic nanoparticles were used to support an ATRP catalyst for the polymerization of methyl methacrylate (MMA) for minimized adverse effect of catalyst support and easy catalyst separation. The nanoparticle-supported catalyst mediated a living/controlled radical polymerization of MMA as effectively as unsupported catalysts. With the addition of 22 mol % of CuBr2, the polymer molecular weights were well-controlled with an initiator efficiency of 0.85 and polydispersity lower than 1.2. The supported catalysts could be easily separated/isolated using an external magnetic field and reused with slightly decreased activity but further improved control. The activity of the recycled catalyst was regenerated by copper metal or in-situ regeneration using reducing agents such as alkylamine or tin(II) compounds. Block copolymerization using PEG macroinitiator and chain extension were conducted to confirm the livingness. Block copolymers with controlled molecular weights and low polydispersity were obtained using the fresh or reused supported catalysts. It was concluded that nanosized supports had reduced adverse effects on catalysis.