We have investigated dielectric properties in Mott insulators GdMnO3 and TbMnO3 under magnetic fields and external quasihydrostatic pressures. In the case of GdMnO3, thermal hysteresis for dielectric constant ε and discontinuous lattice distortion were observed at ferroelectric transition temperature (TC), and ferroelectric spontaneous polarization was suppressed by the application of external pressure. These results indicate that the ferroelectric transition in GdMnO3 is a first-order displacive-type one. On the other hand, the thermal hysteresis and discontinuous lattice striction were not observed at TC in TbMnO3. The peak of ε corresponding to the ferroelectric transition was shifted toward higher temperatures by the application of external pressure in TbMnO3. The ferroelectric transition of TbMnO3 was thought to be a second-order order–disorder-type one.