When particles in an electrolyte subjected to an external electric field get close to each other, the presence of particles could alter the local electric field and consequently induce mutual dielectrophoretic (DEP) forces on each other. In this paper, a transient, two-dimensional (2D) multiphysics model taking into account the particle-fluid-electric field interactions under a thin electrical double layer (EDL) assumption is performed to investigate the effects of the imposed electric field, the initial particle's orientation and distance on the DEP particle-particle interaction between a pair of micro-sized particles and their relative motions. Prior to the study of the DEP particle-particle interaction, the magnitude comparison between the DEP particle-particle interaction and the Brownian motion is analyzed. When the DEP particle-particle interaction dominates the random Brownian motion, it is expected to observe the particle chaining along the direction of the imposed electric field, independent of the initial particle orientation. The numerical predictions are in qualitative agreement with the experimental observations available from the literature. During the attraction motion of particles, their velocities tend to dramatically decrease due to the rapid increase in the repulsive hydrodynamic pressure force when the particle distance decreases to a certain value. One exclusive exception of the particle chaining occurs when the initial connecting line of the particles is perpendicular to the imposed electric field, which is extremely unstable owing to the inevitable Brownian motion. (C) 2010 Elsevier Inc. All rights reserved.