Leaf venation of all the 20 species of Caesalpinia occurring in China were studied. Venation patterns of the leaflets in the genus studied are all brochidodromous. The secondary veins are abruptly curved. The highest vein order is quaternary or quinternary. Veinlets are simple, or once or twice branched. Leaf venation characters show that both Caesalpinia s.s. and Mezoneurum are in a natural group. A key to the species of Caesalpinia based on leaf venation characters is presented.