In this work, we study the sub-wavelength imaging properties of a plano-convex nano-lens consisted of alternatively arranged dielectric and metallic thin layers. The thickness of one layer is not uniform, and the curvatures of the layer boundaries vary gradually from layer to layer. The image resolution of this nanolens at wavelength 365 nm is about 40 nm, far below the diffraction limit, and its image magnification is much better than that of the original proposed hyperlens of the same size. We also study the imaging properties of the corresponding 3D structures. Numerical simulations reveal that the image recognition ability of the 3D nanolens can be controlled by changing the polarization of the source waves. Devices of this kind may find applications in biological morphology and nano-structured materials researches.