A strategy is presented to acquire active terahertz (THz) metamaterial by incorporating negative differential resistance carbon nanotubes. Furthermore, we propose a method of extracting active metamaterial effective parameters. This new method can effectively solve the problems of signs and multi-branches, while the traditional parameter extraction method becomes powerless for active case. Our results indicate that the active THz metamaterial with metal wires array not only has negative value of the imaginary part of the permittivity but also presents magnetic-dispersion characteristics.