Deep level defects in as-grown and annealed n-type and semi-insulating InP have been studied. After annealing in phosphorus ambient, a large quantity of deep level defects were generated in both n-type and semi-insulating InP materials. In contrast, few deep level defects exist in InP after annealing in iron phosphide ambient. The generation of deep level defects has direct relation with in-diffusion of iron and phosphorus in the annealing process. The in-diffused phosphorus and iron atoms occupy indium sites in the lattice, resulting in the formation of P anti-site defects and iron deep acceptors, respectively. The results indicate that iron atoms fully occupy indium sites and suppress the formation of indium vacancy and P anti-site, etc., whereas indium vacancies and P anti-site defects are formed after annealing in phosphorus ambient. The nature of the deep level defects in InP has been studied based on the results.