Purpose: The purpose of this study was to examine the effects of a simulation-based educational program on anxiety and discomfort in patients with early stage of stomach cancer or stomach adenoma undergoing gastroendoscopic surgery. Methods: The study employed a non-equivalence control group and non-synchronized design to prevent diffusion of treatment. Data were collected from 110 patients (experimental group=55, control group=55) admitted to a department of gastroenterology to undergo gastroendoscopic surgery for early stage of stomach cancer or stomach adenoma. The program was conducted as a 30-minute simulation-based education including 12 processes which patients would experience. Outcome variables were state anxiety, objective discomfort, and subjective discomfort. Results: The scores of state anxiety in the experimental group significantly decreased over time compared to those in the control group. The scores of objective and subjective discomfort after surgery in the experimental group were significantly lower than those in the control group. Conclusion: The study findings suggested that the simulation-based educational program making a person feel sense of real experiences could reduce anxiety and discomfort in gastroendoscopic surgery patients.