The design of new types of metamaterials and the development of applications require good knowledge of their electromagnetic properties. Many characterization methods have been proposed to find the permittivity and permeability but due to the particular structure of these materials some key considerations must be taken into account. In this work an "in-situ" method for obtaining the complex parameters is presented. The measurement cell consists in a stripline which produces the adequate conditions identified for the characterization of this type of structures. The advantages of using this cell, its electromagnetic analysis and some experimental and simulated results are presented.