A novel microstructured-core optical fiber with large mode area and flat-topped mode field distribution is proposed. The core of the fiber is composed of high refractive index dielectric cylinders with a non-uniform distribution. Numerical simulation on the mode area, mode field distribution of the fundamental mode and other aspects of the fiber indicates that the fiber can effectively obtain flat-topped mode field distribution by tuning the parameters of the cladding and the core of the fiber. In addition, the flat-topped fiber has a large mode area of 1655 μm2 at the operating wavelength of 1064 nm.