The spin-polarized tunneling current transport through a ferromagnetic GaMnN resonant tunneling diode is investigated theoretically. Two distinct spin splitting peaks can be observed in the current-voltage characteristic. Spin splitting peaks and the spin polarization decrease and then disappear with increasing temperature. When charge polarization effect is considered for the GaN heterostructure, the spin-down resonant current peak becomes enhanced significantly and spin polarization is also increased accordingly. A highly spin polarized current can be obtained at a certain polarization charge.