We study the properties of tunable nonlinear metamaterial operating at microwave frequencies. We fabricate the nonlinear metamaterial composed of double split-ring resonators and wires where a varactor diode is introduced into each resonator so that the magnetic resonance can be tuned dynamically by varying the input power. We show that at higher powers the transmission of the metamaterial becomes power-dependent,and we demonstrate experimentally power-dependent transmission properties and selective generation of higher harmonics.PACS numbers: 41.20.Jb, 42.25.Bs, Engineered microstructured metamaterials demonstrate many intriguing properties for the propagation of electromagnetic waves such as negative refraction. Such materials have been studied extensively during recent years [1]. Typically, such metamaterials are fabricated as composite structures created by many identical resonant scattering elements with the size much smaller than the wavelength of the propagating electromagnetic waves; such microstructured materials can be described in terms of macroscopic quantities-electric permittivity ǫ and magnetic permeability µ. By designing the individual unit cell of metamaterials, one may construct composites with effective properties not occurring in nature.Split-ring resonators (SRRs) are the key building blocks for the composite metamaterials, in particularly the materials having the negative refractive index [2]. Recent theoretical studies have demonstrated how to dynamically tune or modulate the electromagnetic properties of metamaterials [3,4,5,6,7,8] and the fabrication of nonlinear SRRs has been demonstrated by placing a varactor diode [9] or a photosensitive semiconductor [10] within the gap of the resonator. The diode allows the SRR element to be tuned by an applied dc voltage or by a high-power signal as was shown already in experiment [9,11]. These recent advances open a way for both fabrication and systematic study of nonlinear tunable metamaterials which may change their properties such as the transmission characteristics by varying the amplitude of the input electromagnetic field.It was shown theoretically that nonlinear metamaterials can demonstrate many intriguing features such as unconventional bistability [3,12], backward phase-matching and harmonic generation [13,14,15], as well as parametric shielding of electromagnetic fields [16]. Some of these features have already been experimentally observed in nonlinear left-handed transmission lines which are model systems allowing for combining nonlinearity and anomalous dispersion [17,18,19]. Moreover, first results on harmonic generation in infrared were reported in Ref. [20]. Importantly, in such composite structures the microscopic electric fields can become much higher than the macroscopic electric field carried by the propagating electromagnetic waves. This provides a simple physical mechanism for enhancing nonlinear effects in the resonant structure with the left-handed properties. Moreover, a very attractive goal is to create tunable metamater...