Abstract: Poly(3, 4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is a promising candidate for terahertz (THz) devices. Its dielectric properties can be changed by the dopant dimethylsulfoxide (DMSO). In this study, the dielectric properties of DMSO-doped-PEDOT:PSS films were investigated using terahertz time-domain spectroscopy (THz-TDS) measurements. The values of the carrier density and mobility were also determined by fitting the dielectric permittivity to the Drude-Smith model. The properties of PEDOT:PSS films can be changed by adjusting the doping concentration. When the doping concentration is increased to 15 vol%, the film exhibits metal characteristics, while the pristine and low-doped film still exhibits semiconductor characteristics.