A spin-polarized rf current injected into a magnetic cell exerts a torque to the local spin momenta and may excite ferromagnetic resonance (FMR) modes in the magnetic cell. FMR mode excitation in a magnetic tunnel junction is accompanied by the oscillation of its resistance and results a rectification effect. This “spin-torque diode effect” provides a quantitative measure of the spin-torque. By using this effect, the origin of the spin-torque and the critical voltage of spin-transfer magnetization switching were investigated. It is predicted that if the critical voltage becomes smaller than 25 mV, the spin-torque diode may have higher rectification output than p-n junction semiconductor diodes at room temperature. Also, the interplay between the giant tunneling magnetoresistance effect and the spin-torque will result in a negative differential resistance effect, and the magnetic tunneling junction may possess an amplification function.