Purpose: This study was designed to identify levels of job stress and assertiveness behaviors and the relationship between them among ICU nurses. Methods: A correlational descriptive survey was used to these relationships among 454 ICU nurses from 25 hospitals which were members of Korean Association of Critical Care Nurses. Data were collected using a structured questionnaire which included 58 items of job stress and assertiveness scale from November 4 to 14, 2008. Descriptive statistics, Pearson correlation coefficients were used to analyze the data. Results: ICU nurses experience mild to moderate degree of job stress, A score of assertiveness behaviors was 2.86. There was a negative relationship between job stress and assertiveness behaviors(r=-.106, p=.024) as influenced by role conflict, lack of professional knowledge and skills, and inappropriate treatment. Conclusion: ICU nurses, especially nurses with 3-6 year clinical experience, are in need of support to manage their job stress. These nurses should be supported through provision of adequate stress management and assertiveness program.