A metalens consisting of GaN nanopillars as basic cells was designed for subwavelength focusing at visible wavelengths, which provided an improvement over bulky and low-efficiency conventional imaging systems, and the possibility of its application to micro imaging. The metalens is composed of a GaN nanopillar array, which maintains the same height but gradually varying width. An analysis of the capacity and reasons for GaN nanopillars controlling the phase of incoming light was performed. In addition, based on FDTD methods, a simulation of 460 nm wavelength focusing in the transmission mode was performed. Subsequently, full width at half maximum (FWHM) are shown when the sizes of metalens are 3.75 μm× 3.75 m, 6.75 μm×6.75 m, 8.75 μm× 8.75 m, and 10.75 μm× 10.75 m, and the results are 1, 0.8, 0.5, and 0.3 m, respectively. The other focusing results, such as intensity distribution and focus spots, were also discussed. As a result, there are differences between designed focal length and simulated focal length, which changes under the different sizes of metalens. In conclusion, the designed GaN based metalens can focus in micron dimension effectively, which reduces the complexity of traditional imaging systems.