Bonding characteristics of liquid boron at 2500 K are studied by using high-resolution Compton scattering. An excellent agreement is found between the measurements and the corresponding Car-Parrinello molecular dynamics simulations. Covalent bond pairs are clearly shown to dominate in liquid boron along with the coexistence of diffuse pairs. Our study reveals the complex bonding pattern of liquid boron and gives insight into the unusual properties of this high-temperature liquid.