Immigration transition may be an opportunity for physical, psychological, and social improvement, but the health of transitioning individuals may actually be at an increased risk for decline. In order to overcome negative influencing factors on the health of immigrants trying to integrate into a new society, examining the impact of transitioning immigration on the health of immigrants is important. Transition has complex and multidimensional patterns based on an individual’s social and cultural background. Korean immigrants in transitioning to the U.S. have experienced a specific and unique situation. Thus, the primary purpose of this study was to identify the impact of immigration transition, which is defined in the middle-range theory of transition, on the health-promoting behaviors and mental well-being of Korean immigrants in the U.S. A cross-sectional, descriptive correlational comparative design was used for examining a total of 192 Koreans: 105 in the U.S. and 87 in S. Korea, which were selected through matched age and gender controls with Korean immigrant participants in the U.S. The average ages were 46.8 (Median = 47, SD = 12.5) of Korean immigrants in the U.S and 46.2 (Median = 46, SD = 12.7) of 87 native S. Koreans. Korean immigrants had a low level of acculturation and limited English proficiency. The level of health-promoting behaviors of Korean immigrants was higher than that of S. Koreans, especially in subscales of health promotion, nutrition, and safety. Social resourcefulness was a key predictor of health-promoting behaviors and mental well-being among Korean immigrants. Also, self-control was a dominant mediator on the relationship between behavioral acculturation and mental well-being. Increasing acculturation and English ability for Korean immigrants, as well as increasing self-control, family functioning, and social resourcefulness were found to be important to improve integrating Korean immigrants into the U.S. These findings provide essential information that all health care professionals can use to increase their awareness of the importance of appropriately treating individuals with different cultural perspectives as well as diverse populations coming from varied countries. / text