In this article, we have theoretically proposed and numerically analysed a resonant wavelength tunable Fabry Perot (FP) filter without changing the physical length of the cavity in the mid-infrared range. The proposed FP filter consists of a multilayer cavity, which is constructed by alternately repeating two different sub-wavelength dielectric or semiconductor materials. Such a multilayer structure can behave as an anisotropic metamaterial. By varying the fill factor of the metamaerial, the effective refractive index of the cavity can be easily tuned. As the resonant wavelength of such a filter strongly depends on the cavity refractive index, the resonant wavelength of the filter can be easily tuned between a wide wavelength range of approximately 400nm by changing the fill factor. Distributed Bragg reflectors (DBRs) have been used as the high reflectivity mirrors. Both the DBRs and the cavity can be constructed by using only two materials, such as Silicon (Si) and SiO 2 . This device may find substantial applications in the fields of spectroscopy, color filtering, sensing etc.