In this paper, nitrogen-vacancy centers in diamonds are created by high-energy electron irradiation and vacuum annealing. The macroscopic color changes and internal defect transformations in those diamonds before and after irradiation and annealing are investigated. The effects of irradiation dose and vacuum annealing temperature on the yield of nitrogen vacancies are discussed. The results show that the concentration of nitrogen-vacancy centers rises and then drops as irradiation dose increases. This is because under a higher irradiation dose, vacancies are more likely to form vacancy clusters, which do not constitute nitrogen-vacancy centers during the annealing process. The concentration of nitrogen-vacancy centers increases with the increasing annealing temperature and then saturates in the temperature range of 800-900 ℃. As the temperature further increases, the concentration declines due to the interaction of nitrogen vacancies with interstitial atoms generated during the irradiation process under a high temperature.