Internal rotational levels of the methyl group in o- and m-toluidine cations have been observed by pulsed field ionization-zero kinetic energy photoelectron spectroscopy. Level energies and transition intensities were reproduced by a one-dimensional rotor model with a free-rotor basis set, and the potential curves of the internal rotation in the cations have been determined. Analysis for m-toluidine shows a drastic increase of the barrier height for internal rotational motion from the neutral to the corresponding cation. On the other hand, the barrier in o-toluidine slightly decreases by ionization. The mechanism of the change in barrier height will be discussed in terms of geometrical and electrical change by ionization. It is suggested that the drastic change of internal rotational motion is mainly determined by the electronic structure.