Microfluidic lenses that have been fabricated using PDMS have serious limitations due to the high gas permeability of the material, which leads to evaporation of the filling liquid and air bubble formation. We report a fast and reliable method of fabricating flexible, thin-film parylene microfluidic devices, and their application in tunable microfluidic lens arrays. A parylene microfluidic lens was built by molding and bonding 1.4-mu m-thick parylene layers using nano-adhesivelayers deposited by initiated chemical vapor deposition (iCVD). The focal length of the lens could be tuned by changing the radius of curvature of the lens and/or changing the refractive indices of the filling liquids. The parylene microfluidic lens showed a large range of focal length tuning, and high reproducibility over hundreds of cycles.