A seamless hexagonal h-BN atomic monolayer thin film has a pseudo-single crystal structure including a plurality of h-BN grains that are seamlessly merged. Each of the h-BN grains has a dimension in a range from about 10 μm to about 1,000 μm. The seamless hexagonal boron nitride (h-BN) atomic monolayer thin film may be fabricated by a process including pre-annealing a metal thin film at a first temperature in a chamber while supplying hydrogen gas to the chamber; supplying nitrogen source gas and boron source gas to the chamber; and forming the seamless h-BN atomic monolayer thin film having a pseudo-single crystal atomic monolayer structure having a grain dimension in a range from about 10 μm to about 1,000 μm by annealing the pre-annealed metal thin film at a second temperature.