Purpose: This study was correlation study to identify the factors influencing sleep quality, fatigue, and postnatal depression in mothers who have given birth during the past 6 months. Methods: The study was conducted using a survey with questionnaires to 329 mothers who visited E University Medical Center, or three local clinics located in D city, between August and October 2013. Collected data were analyzed using the SPSS/WIN 20.0 program. Results: Out of 329 subjects, 18.2% showed that they had mild postnatal depression whereas 24.3% had severe postnatal depression. Accordingly, 42.5% reported having postnatal depression. Postnatal depression had a significant correlation with sleep hours after childbirth (r=-.16, p=.003), spousal support (r=-.28, p<.001), sleep quality (r=-.35, p<.001), physical fatigue (r=.66, p<.001), psychological fatigue (r=.69, p<.001), and neurosensory fatigue (r=.56, p< .001). Factors influencing postnatal depression include psychological fatigue, sleep quality, number of child births, and neurosensory fatigue, and these accounted for 53% of postnatal depression. Conclusion: Results indicate that factors influencing postnatal depression involve psychological fatigue, sleep quality, number of child births, and neurosensory fatigue. Therefore for nursing intervention for postpartum mothers, it is necessary to assess the level of depression, fatigue, and sleep quality, and to provide interventions to relieve depression.