The effects of an applied magnetic field on the system composed of polyelectrolytes (PEs) and magnetic nanoparticles oppositely charged are studied by means of Monte Carlo method within the framework of single-site bond fluctuation model. For a certain concentration of chains, the coil-globule transition can be induced by the applied magnetic field. The mean-square end-to-end distance and gyration radius as well as the shape factor of PE chains are used to characterize the conformational transitions. The statistical analysis of the system energy demonstrates this significant physical process. The role of entropy-energy balance is well-understood for different chain lengths, and a typical phase-transition anomaly concerned with specific heat curve is observed. Under a certain magnetic field, the PE chains will regularly collapse due to the enough adsorption of magnetic particles. The magnetic particles exhibit peculiar spatial distribution at high magnetic fields: the string-like arrangement along the magnetic field and the square lattice-like arrangement perpendicular to the magnetic field. The applied magnetic field has a great influence on the length of string-like structures formed by nanoparticles. This investigation may cast light on the collapse of PEs and provide a promising method for producing new nanocomposites. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012