We propose an engineered microfiber with nano-scale slots that produce ultra-flattened and low dispersion of ±10 ps/(nm · km) over a 340 nm wavelength range. It is comparable with the results in photonic crystal fibers and planar slot waveguides, but can be hardly realized in conventional circular microfibers. By confining the light in a low nonlinearity air slot, the nonlinear coefficient can be greatly reduced. With the unique geometry and excellent performance, the slot microfiber offers large potential in miniature fiber devices for high-speed telecom applications.