The objective of this work is to investigate the possible effects of plate incline on the performance of a small plate Differential Mobility Analyzer (DMA) in which two separate metal plates are installed in parallel (with constant spacing). Plate incline may occur due to improper plate DMA assembly, which raises the concern of adverse DMA performance, particularly for small DMAs with narrow plate-to-plate spacing. COMSOL Multiphysics 5.4 (R) was used to numerically study the following parameters: the relative incline, the incline type (sided and pointed), the aspect ratio and classification length of the particle classification channel, the particle slit opening length (relative to the channel width), the sheath-to-aerosol flow rate ratio, and the total flow rate. Our study shows that the plate incline results in the deterioration of the DMA transfer function (i.e., peak mobility shifting, peak height reduction, and width-at-half-peak-height broadening), primarily attributed to the distortion of the electrical field. The effect of plate incline on the DMA transfer function manifested as either a decrease in the total DMA flow rate or an increase in the classification channel length in a plate DMA. It was concluded that the observed effects were due to increased particle residence time and the distorted electrical field acting on particles in the DMA classification channel. Additionally, the observed adverse effects were found to be sensitive to the particle-slit opening length.