Three identical hydrothermal self-assembly reactions in a mixture of nickel nitrate hexahydrate, nicotinic acid, sodium azide were carried out with the applied magnetic fields of 0, 0.15, 0.3T, respectively. It was found that [Ni-1.5(N-3)(nic)(2)(Hnic)](n), can be obtained at 160 degrees C for 48 h without an applied magnetic field, while the final product totally changed into [Ni-2(nic)(4)(H2O)] as a 0.3T weak magnetic field was employed in the reaction. Both of the two products show three-dimensional frameworks, however, they display two different coordination geometries with different ligands. We consider the mechanism base on changes of magnetic susceptibilities and structures of [Ni-2(nic)(4)(H2O)], and propose a view that the magnetic field can reduce the activation energy of reaction pathways according to transition state theory. The possible reason of the decrease of activation energy is that a magnetic field can mediate coupling among the Ni2+ ions during the formation of [Ni-2(nic)(4)(H2O)], which is indicated by the slight changes of microstructure of crystal, such as bond angle and length. (C) 2012 Elsevier B.V. All rights reserved.