Background: Most postpartum mothers experience fatigue. However, it was unclear which day after birth during hospitalization mothers are the most fatigued, and if the mothers' background is a factor. The aim of this study was to investigate hospitalized mothers' characteristic features, fatigue, and relaxation, and then to find the correlated factors of fatigue in order to obtain suggestions for mothers' early postpartum care. Methods: This quantitative cross-sectional descriptive correlational study occurred during May to December 2016. A purposive sample of early postpartum hospitalized mothers at a postpartum unit in a Tokyo hospital completed an anonymous self-filled questionnaire survey. The survey included respondents' characteristics, fatigue scale, and relaxation scale. Data were analyzed with descriptive statistics, independent t-test, one-way analysis of variance, multiple regression analyses, and structural equation modeling. Results: Returned questionnaires were 251 (83.4%). Fatigue was significantly higher for mothers with longer postpartum days compared to shorter postpartum days (F = 5.7, p < 0.001). Fatigue was significantly higher in mothers with a previous disease compared to no previous disease (t = 2.2, p < 0.05). The three significant predictors of Fatigue were 1) relaxation (β = −0.30, p < 0.001), 2) postpartum days (β = 0.26, p < 0.001), and 3) previous disease (β = 0.14, p < 0.05). The structural equation modeling revealed that the same factors were related to Fatigue. Conclusions: No relaxation, longer postpartum days, and previous disease were associated with Fatigue for early postpartum mothers. These results suggest that focusing on previous disease, length of postpartum days, and mother's relaxation is needed to provide full support to early postpartum mothers.