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The evolution behavior of the γ″ phase of IN718 superalloy in a temperature/stress coupled field was investigated. Results showed that the coarsening rate of the γ″ phase was significantly accelerated in the temperature/stress coupled field. Based on the detail microstructural and crystal defect analysis, it was found that the coarsening rate of the γ″ phase with applied stress was significantly higher than that without stress. The main reasons for the increase in the coarsening rate of the γ″ phase are as follows: the vacancy formation energy is decreased by the applied stress, which leads to an increase in the vacancy concentration; in the temperature/stress coupled field, the Nb atoms easily combine with vacancies to form complexes and diffuse with the complexes, resulting in a significant increase in the Nb atom diffusion coefficient; Nb atom diffusion is the key control factor for the coarsening of the γ″ phase.