We report pseudopotential density-functional calculations of the “3 ×1” phase of the Ba/Si(111) surface. To resolve the coverage issue of the Ba/Si(111) surface, we investigate various structural models with two Ba coverages: 1/3 ML and 1/6 ML. Based on the comparison of the simulated STM images for two coverages and the experimental images, it is found that the “3 ×1” phase has 1/6 ML Ba and a 3 ×2 structure with the honeycomb chain-channel Si reconstruction. The substrate reconstruction is quite similar to the Si(111)-3 ×1 surface induced by 1/3 ML alkali-metals. The experimental semiconducting band gap is well reproduced by the LDA band structure.