A series of FeNi-MgO nanogranular soft magnetic films with different sputtering powers of MgO were deposited by co-sputtering at room temperature on naturally oxidized silicon substrates. By adjusting the sputtering power of MgO, the average grain size of FeNi-MgO granular film reduces from 6.5 to 4 nm. The easy-axis coercivity H-ce decreases from 8 to 0.3 Oe and saturation field H-s decreases from 14 to 1 Oe. When the sputtering power of MgO is above 60 W, H-ce and H-s keep nearly unchanged. The results of permeability spectra indicate that the resonant frequency f(r) can be modulated in a small range from 0.61 to 0.71 GHz. Meanwhile, Gilbert damping alpha first decreases and then increases with increasing the sputtering power of MgO. The initial permeability is higher than 500 when the frequency is below 0.5 GHz, indicating that the FeNi-MgO nanogranular films are the excellent candidate for a high-frequency application.