Purpose: The purpose of this study was to (1) develop an acceptance commitment therapy (ACT) based recovery enhancement program for inpatients with mental illness and (2) test the effects of the program on patients’ psychological flexibility, recovery attitude, and quality of life. Methods: A mixed methods design was used: a combination of a repeated-measure design with a non-equivalent control group and qualitative data collection. The participants were 41 inpatients with mental illness (experimental group: 20, control group: 21). The ACT based recovery enhancement program was conducted over four sessions for a total of two weeks. Data were collected from December 2018 to January 2019. Results: Study results revealed that the ACT based recovery enhancement program was effective for psychological flexibility (F=150.71, p<.001), recovery attitude (F=60.22, p<.001), and quality of life (F=31.59, p<.001) for inpatients with mental illness. Conclusion: The results of this study suggest that the ACT based recovery enhancement program can be used as an effective mental health nursing intervention for psychological flexibility, recovery attitude, and quality of life of inpatients with mental illness.