The structural phase transition and metallization of the simple alkali iodide, sodium iodide (NaI) is investigated through their band structures. The band structure, density of states (DOS) and total energy are computed as a function of volume for both NaCl and CsCl phases using the full potential linear muffin-tin orbital (FP-LMTO) method. The phase transition pressure (P T ) and the corresponding reduced volume (V T ) estimated in our calculation are 0.038 Mbar and 0.89 respectively. The metallization pressure P M is 2.6259 Mbar and the corresponding reduced volume (V M ) is 0.36. The results of the metallization pressure in NaI are compared with that of other alkali iodide spotassium iodide (KI) and RbI. It is found, that the charge transfer from s and p states to the d state will cause metallization and the metallization pressure increases with a decrease of the lattice constant. It is also confirmed that the metallization and structural phase transition do not occur simultaneously in ionic compounds.