Objectives: The present study evaluated the relationship between emotional labor and mental health among nurses in Catholic hospitals in Taiwan. Methods: Based on a cross-sectional and correctional study design, the researcher invited nurses from six Catholic hospitals to participate between July 1 and October 30, 2010. Instruments were the Emotional Labor Scale developed by Shang Ping Lin and the Self Reporting Questionnaire 20 (SRQ-20) developed by the World Health Organization. Results: A total of 653 subjects were invited to participate, and 624 completed questionnaires were returned for an effective response rate of 95.56%. By using descriptive statistics, ANOVA, correlation, and regression analysis in SPSS 18.0, we found that: (1) Subjects had a relative higher degree of emotional labor (M ± SD = 5.41± 1.19) and poorer mental health status (M ± SD = 2.81 ±1.05) as measured by the SRQ-20; of 50.4; (2) There were significant differences in rules for emotional display, superficial acting, and mental health among different groups based on educational level, position, and working department; (3) Age and service seniority had a highly positive correlation with rules for emotional display, superficial acting, and mental health, whereas age was positively associated with deep acting; (4) Rules for emotional display rule, superficial acting, frequency and duration of interaction, and deep acting were highly correlated with mental health; (5) Emotional labor accounted for 21% of the variance in mental health. Conclusions: Nursing staff bear a high level of emotional labor with unsatisfactory mental health status. Administrators should pay close attention to these issues.