The objective of this study is to investigate the effect of inner rod tilt on the transfer function of a cylindrical differential electrical mobility classifier (DEMC), a key component in differential mobility sizers. Numerical modeling was applied in this study. It is found that the inner rod tilt resulted in the variation of flow and electrical fields in the DEMC classification channel, consequently deteriorating the DEMC transfer function (i.e., reduced peak and widen the width). In high-tilting cases, the single peak of the DEMC transfer function was split into two. Further investigation found that the alternation of electrical field in the classification channel (due to the rod tilting) is the primary contributor to the transfer function deterioration. It is also found that the effect of rod tilt on the transfer function is very pronounced when a DEMC operates at a high sheath-to-aerosol flow rate ratio. More, the increase of total DEMC flow reduced the adverse effect of rod tilt on the DEMC transfer function and the increase of both the radii ratio, R2/R1, and classification channel length, L, negatively impacts the DEMC transfer function.