The pattern of undulation on the equatorward boundary of the diffuse aurora, occasionally observed in the afternoon-evening sector, is studied by two-dimensional particle simulations for the motion of magnetospheric plasma perpendicular to the geomagnetic field. In the simulation model, the poleward electric field is assumed to be initially enhanced locally in latitude and independent of longitude, as suggested by the observations near the undulations of the diffuse auroral boundary. Such an electric field is assumed to be maintained by space charges carried by a cold plasma. The diffuse aurora is initially distributed over a longitudinally extended zone. Since this type of diffuse aurora is probably caused by precipitation of the energetic protons with energies in excess of a few keV, the temporal evolution of the diffuse auroral pattern, as displayed in photographs from the satellites, can be visualized by following the cluster of the energetic protons. Sheared plasma flow arising from the electric field distribution drives the Kelvin-Helmholtz instability and causes undulations on the equatorward boundary of the diffuse aurora with wavelengths and amplitudes comparable to the typical observed values. Notably, the particle simulation can reproduce the outstanding indentations characteristic of large-amplitude undulations which have occasionally been observed. The good agreement between the simulation results and the observed undulations suggests that the Kelvin-Helmholtz shear-driven process is responsible for undulations on the equatorward boundary of the diffuse aurora in the afternoon-evening sector. It is also shown that the magnetic drift of the energetic protons producing the diffuse aurora is important in creating the characteristic pattern of the diffuse auroral boundary. ¿ 1991 American Geophysical Union