The atomic and electronic properties of the substitutional phosphorus (P) on the Si(1 1 1)-(2x1) surface have been studied by using the ab initio density functional theory (DFT) based on pseudopotential approach. We have considered four different possible binding sites for P adatom in the pi-bonded chain labeled sites 1-4 respectively in Figure 1. We have found that the site 1 position in the pi-bonded chain was energetically more favorable than the other binding sites, by about 0.1 eV/adatom. We have also calculated the corresponding surface electronic band structure and found one surface state, labeled C, in the fundamental band gap of Si(1 1 1)-(2x1) surface. Our calculations show that the P/Si(1 1 1)-(2x1) surface has a metallic character in the nature. In order to explain the nature of this surface state in the bonding geometry, we have depicted the total and partial charge density contours plots at the (J) over bar point of the surface Brillouin zone (SBZ).