Purpose: This study was done to analyze core competencies affecting critical thinking ability of student nurses. Core competencies investigated in this study were problem solving ability, communication ability and self-directed learning ability. Method: Data were collected from a convenience sample of 322 student nurses in 2 provinces during the period from May 21 to June 8, 2007. Critical thinking, problem solving ability, communication ability and self-directed learning ability were measured using the Disposition towards Critical Thinking Scale by Park(1999-a) and the Core Competencies Scale by Lee(2003). Descriptive statistics and correlation coefficients with the SPSS WIN 12.0 program were used to analyze the data. Results: There were significant differences in the critical thinking according to grade, type of high school, experience with PBL, preference for lecture-based learning, preference for discussion and perceived logicality. The score for critical thinking showed significantly positive correlations with scores for problem solving ability(r=0.54, p=0.00), communication ability(r=0.56, p=0.00) and self-directed learning ability(r=0.54, p=0.00). Conclusion: The results of this study suggest that problem solving ability, communication ability and self-directed learning ability are significant factors affecting critical thinking in student nurses.