In this work, the impedance spectroscopy was adopted to characterize conductively doped organic hole-transport layers. We performed comparative studies of the impedance spectroscopy of undoped and doped hole-transport materials, by both experiment and simulation approaches. The impedance spectroscopy of the non-doped hole-transport material can be well understood by simply adopting the conventional RC equivalent circuits and considering the dielectric response frequency-independent. For the conductively doped organic hole-transport materials, however, successful modeling of the impedance spectroscopy results need to take into account the more complicated situations: including the difference between the bulk region and the depletion region near the electrode, and dispersion (i.e. frequency dependence) in the dielectric response of the conductively doped transport layer. As such, it is found necessary to include the complex dielectric response and the non-conventional complex impedance element in the equivalent circuit to achieve tight matching of simulated results with experiment results over wide bias and frequency ranges.