In this paper, we explore building planar transmission line from carbon nanotube (CNT) networks. We are successful in fabricating the transmission line and verifying the feasibility of potential planar transmission lines where carbon nanotube networks replace the metallic lines. The experimental realization and the two-port microwave measurements enabled us to extract accurately the fundamental parameters of the proposed transmission line. The frequency-dependent phase velocity characteristics show clearly its dramatic reduction compared to speed of light in vacuum. The complex permittivity of CNT networks is also reported in our work.