A tunable metamaterial is proposed by combining a thermochromic oxide with a fishnet structure. The reflection and transmission coefficients are calculated by finite-difference time-domain (FDTD) method. The effective electromagnetic parameters of the metamaterial are retrieved on the basis of these data. The results reveal that an effective negative refractive index is obtained by this proposed structure and the wavelength region with negative refractive index can be self-regulated by simply tuning the temperature, which is of importance to extend the applications of negative refractive index materials. The effects of structural sizes on the negative refractive index are discussed. The size-dependence indicates that the wavelength range in which the apparent refractive index is negative can be tuned to be located at the desired position by dexterously tailoring the structural parameters.