The charge carriers conduction mechanism of single-layer device ITO/Alq3(90 nm)/Al has been studied by impedance spectroscopy. According to the frequency-dependent response and its equivalent circuit model，the device can be interpreted in terms of the equivalent circuit model of a contact resistance Rs in series with a parallel combination of a capacitance Cp and a resistance Rp, and the values of Rp, Cp and Rs can be found from experimental data. Our experimental results confirmed that the charge carriers conduction mechanism of ITO/Alq3(90 nm)/Al is trap-charge limited current with exponential distribution, and the dielectric relaxation time of the material decreases dramatically with increasing applied bias voltages.