The negative differential resistance entirely due to X-band electron tunneling through the X-band profile in an AlAs/GaAs/AlAs/GaAs/AlAs diode which uses the AlAs as the X-band electrodes and tunneling well and the GaAs as the X-band barriers is observed. Two small inflection points in the current-voltage curve are measured at 8.8 K at low biases (24 and -48 mV) which disappear above 30 K. These peaks are found to be due to the X(l) (normal to the interface) electrons in the AlAs electrode tunneling through the quasiquantized state of X(l) valley in the AlAs well. Two tunneling peaks with negative differential resistance are also observed at 8.8 K which disappears above 100 K. The tunneling peak current density is 3.1 A cm-2 at a bias of -0.18 V and the peak to valley ratio is 1.4. This peak is attributed to the X(t) (parallel to the interface) electrons in the AlAs electrode tunneling through the quasiquantized state of X(t) valley in the AlAs well.