Introduction: Cancer ranks as a leading cause of death and an important barrier to increasing life expectancy in every country of the world. The purpose of this study was to investigate the mediating role of mental adjustment in the relationship between cognitive processing with post-traumatic stress symptoms and fear of recurrence in patients with cancer. Methods: This is a cross-sectional study with a convenience sampling method. A total of 200 women and man with one of the types of cancer (breast, blood and gastrointestinal) in Tehran, Iran participated in this study and completed five questionnaires, namely the PTSD checklist, fear of cancer recurrence scale, and mental adjustment to cancer scale. The findings were analyzed through the advanced statistical method of structural equation modeling and using SPSS26 and SmartPLS 4 software Results: Structural Equation Modelling demonstrated that cognitive processing was negatively associated with PTSD (β, =-. 473,T=11. 84,P<. 001) and fear of recurrence (β, =-. 462,T=10. 52,P<. 001), also mental adjustment was negatively associated with PTSD (β, =-. 496,T=11. 84,P<. 001), and fear of recurrence (β, =-. 462,T=10. 52,P<. 001), and cognitive processing were positively associated with mental adjustment (β, =. 753,T=34. 52,P<. 001). Bootstrapping analysis showed that mental adjustment acted as a mediator between cognitive processing and PTSD, and cognitive processing and fear of recurrence. Conclusions: Mental adjustment partially mediated the relationship between cognitive processing with PTSD symptoms and fear of recurrence. Future intervention plans should pay more attention to cancer-related cognitive impairment as well as PTSD symptoms, and fear of recurrence in the treatment of patients with cancer.