Large amplitude oscillatory shear (LAOS) behavior of electrotheological (ER) fluids has been investigated using three-dimensional particle-level dynamics simulation. As an ER device usually operates in a dynamic mode with large deformation, it is important to understand the LAOS behavior as well as its underlying mechanism for the development of an effective ER fluid. Simulation predicted most of the experimental observations including strain overshoot, distorted stress signal, dogbone-type Lissajous curve, and stripe pattern formation, to list a few. By careful investigation of cluster statistics as well as microstructures, we found that the strain overshoot phenomenon, which is often observed in complex fluid systems with little explanation, arises from the cluster reformation process in addition to a slight rearrangement within a cluster. Fourier transformation analysis was also performed, and the scaling behavior of the intensities of higher harmonics was investigated. The intensities of higher harmonics were found to increase according to the power of harmonic order. As it becomes more important to understand the nonlinear behavior of complex fluids, it is expected that our results increase our understanding on the complex nonlinear behavior as well as Fourier transformation theology of ER fluids in relation with the microstructural changes. (C) 2003 The Society of Rheology.