The electronic structure of liquid boron and cluster formation in expanded liquid boron have been investigated with first-principles molecular dynamics simulations. The calculated electronic density of states (DOS) exhibits a metallic feature, while liquid boron is known experimentally to be semiconductive. Since the DOS is not very sensitive to density, the electronic states near the Fermi level will consist mainly of dangling bonds, which explains the difference between the calculated and experimental results. Many types of clusters are formed in expanded liquid boron. This formation occurs in a very different way from that at low temperatures because expanded liquid boron has a high temperature and pressure that are close to the liquid-gas critical point. As the density is reduced, the coordination number in boron clusters decreases to about 2, indicating that the cluster geometry tends to be one- rather than two-dimensional, which is the most stable form at low temperatures. In fact, the analysis of small clusters proved that one-dimensional forms are dominant over two-and three-dimensional forms. This is because one-dimensional geometries have a more flexible structure and a high entropy value that consequently reduces the free energy at high temperatures.