Purpose: This study was to describe the relationship between optimism and post-traumatic stress disorder (PTSD) in breast cancer patients. Methods: The data were collected by self-administered questionnaires from March 5 to April 24, 2012. The participants included 204 breast cancer patients at K university hospital in D city. The utilized measurements were Life Orientation Test-revised (LOT-R) and Impact of Event Scale (IES). Results: The mean scores of optimism and PTSD were $21.61{pm}3.54$ and $30.18{pm}10.43$, respectively. There were significant differences in the score of optimism by monthly income, engaging in a hobby, perceived level of stress management, need to manage stress, and people thought to be helpful in stress management. There were significant differences in the score of PTSD by engaging in a hobby, recurrence of cancer, the number of hospitalization, perceived level of stress management, need to manage stress, and people thought to be helpful in stress management. There was a negative correlation between optimism and PTSD. Conclusion: To relieve post-traumatic stress in breast cancer patients, nurses should provide ongoing psychological stress assessment from the beginning of the diagnostic stage and offer proper nursing intervention to improve optimism in dealing with their stress.