Chain-like nanostructures consisted of more than 10 magnetite nanoparticles with an average size of 80 nm were hydrothermally prepared at 200 degreesC under a 0.18 T external magnetic field. Magnetic measurement results revealed that the chain-like structure showed higher saturation magnetization M-s, remnant magnetization M-r. coercivity H-c and reduced magnetization M-r/M-s,M- compared with those of randomly distributed magnetite nanoparticles prepared without the induction of external magnetic field. Possible reasons responsible for the changes of the above magnetic parameters were tentatively discussed. (C) 2004 Elsevier B.V. All rights reserved.