This paper presents a broadband microwave non-destructive technique for characterization of carbon nanotubes (CNTs). Typically, commercially available carbon nanotubes are powder-like samples and, therefore, in this paper, a broadband characterization technique to extract electrical conductivity of powder materials is developed. The technique uses a microstrip line configuration in conjunction with a cavity resonator technique. The electrical conductivity of CNTs is extracted from the measured attenuation in the signal response (|S 21|dB) for the microstrip configuration with a signal trace made of copper and a ground plane filled with CNT samples. A resonant cylindrical cavity is also used in the measurement process to help determine a correction factor for the surface roughness of the CNT microstrip ground plane. A novel method to take attenuation due to surface roughness into account to determine conductivity of CNTs is introduced. Experimental and numerical verification of the proposed method is provided. The method developed in this work provides a cost-effective solution where significant amount of time and cost are reduced in the sample preparation process. Measurement results for the electrical conductivity of single-walled CNTs and multi-walled CNTs are presented.