Pure rotational transitions of MgO in the a 3 Π i state and the ground X 1 Σ + state (v=1–3) were observed in the 210–400GHz region. The MgO molecule was produced by a DC discharge in a mixture of Mg vapor and N 2 O. A simultaneous deperturbation analysis for the X 1 Σ + , a 3 Π, and A 1 Π states was carried out for the observed transition frequencies including previous high-resolution data, where the energy levels were calculated by diagonalizing the energy matrix including the spin–orbit and orbit–rotation interactions among three electronic states with vibrational levels up to v=9. A centrifugal term of the spin–orbit interaction between the a 3 Π i and X 1 Σ + states was newly introduced to improve quality of the fitting, to explain the observed Λ-type doubling of the a 3 Π i state.