Purpose: The purpose of this study was to identify the mediating effect of stress between smartphone addiction and the sleep quality of Korea Armed Forces Nursing Academy (KAFNA) cadets. Methods: This cross-sectional study recruited 304 cadets and they completed a self-administered questionnaire. The data were collected from August 23 to 29, 2021 and analyzed with descriptive statistics, independent t-tests, one-way ANOVA, Scheffé test, Pearson correlation coefficient, simple & multiple linear regression, and mediation analyis with sobel test. Results: The mean scores of smartphone addiction and stress were 2.17±0.53 and 2.50±0.55, respectively. The mean score of sleep quality was 3.16±0.41 out of 4. In simple linear regression analysis, both smartphone addiction (β=-.267, p<.001) and stress (β=-.432, p<.001) had significant effect on the sleep quality. But in multiple linear regression analysis, stress (β=-.432, p<.001) had significant effect on sleep quality while smartphone addiction (β=-.107, p=.060) did not. Thus, stress demonstrated a complete mediating effect on the effect of smartphone addiction upon sleep quality by Baron and Kenny's method. And the result verified by Sobel test (Z=-5.2, p<.001.) was the same. Conclusion: This study showed that stress completely mediated the effect of smartphone addiction on sleep quality. Based on the result of this study, it is necessary to develop a proper smartphone use and stress management education program to improve KAFNA cadet's sleep quality.