Two identical hydrothermal self-assembly reactions were carried out with and without an applied magnetic field of 0.2T. Two kinds of single crystals can be both obtained in the two reactions. It is shown that the crystal yield of the magnetic material is remarkably increased after the weak magnetic field is employed. We ascribe the magnetic field effect to the decrease of activation energy of antiferromagnetic complex during the course of the self-assembly reaction. And the decrease of activation energy is because the partial magnetic phase transits from paramagnetic phase to antiferromagnetic phase. Moreover, magnetic measurement data also reveal that the antiferromagnetic coupling interaction of the magnetic material has been enhanced. Crown Copyright (c) 2008 Published by Elsevier B.V. All rights reserved.