Abstract: Boron-rich slag could be separated from ludwigite by pyrometallurgical method. Na2CO3 was added to the melted slag to change the existing form of boron, then the Na4B2O5 and NaBO2 in crystalline phase were precipitated from the Na2CO3-modified slag at high temperature as shown by XRD analysis. Sodium borates thus turned into solution via the water leaching, grinding, filtering and crystallizing of Na2CO3-modified slag and, as a result, borax could be precipitated from the solution at room temperature. Effects of the adding dose of Na2CO3, heat-treatment temperature and particle size of Na2CO3-modified slag on the leaching rate of boron were investigated by chemical analysis and XRD. It was found that the actual adding dose of Na2CO3 is 2.3 times as great as the theoretical one if the Na2CO3-modified slag is treated at 650°C and held for 1.5 hr, and that the highest leaching rate of boron can be up to 76.04% if the particle size of Na2CO3-modified slag is less than 74 μm.