The millimeterwave rotational spectrum of o-chlorotoluene is investigated in the frequency region 150-250 GHz. Many rotational lines show splitting due to internal rotation of the methyl group. The analysis of the internal rotation splitting allows us to determine with precision the potential barrier to internal rotation of the methyl group. However, it is found that the moment of inertia of the methyl top is probably much smaller than usually assumed, which significantly affects the value of the barrier. Accurate centrifugal distortion constants are obtained for the ground states of 35Cl and 37Cl isotopologues as well as for an excited vibrational state.