Introduction: Weight retention during postpartum period is generally not preferred by mothers. Mothers are known to reduce energy intake during lactation in order to lose weight. Additional energy is required during lactation to provide for breast milk production. This study aimed at investigating factors that influence postpartum weight loss. Methods: Data from a prospective cohort quasi experiment study conducted in Beji subdistrict, Depok City were used. This study had recruited lactating mothers from community health centres and they were followed up from delivery to six months postpartum. A total of 201 breastfeeding mothers determined by cohort sample size formula were included. The dependent variable was six months’ postpartum weight loss, while the independent variables included age, energy intake, education, working status, parity and exclusive breastfeeding practice. Bivariate analysis using independent t-test and multiple linear regression was used for statistical analysis. Results: Mean age of the mothers was 30.3 years (95% CI: 29.8-31.1 years). Majority of the mothers had senior high school education, were not working, multiparous and practising exclusive breastfeeding. Mean intake of energy was 1946 kcal/day (1897-1994 kcal/day). They experienced a mean weight loss of 3.79 kg during the study period (3.27-4.31 kg). Postpartum weight loss was associated with exclusive breastfeeding (p=0.004), and education status of the mothers (p=0.029). Conclusion: Exclusive breastfeeding for six months is the dominant factor associated with postpartum weight loss of the mother. Breastfeeding mothers should be supported on their intention to exclusively breastfeed and not be unduly concerned with gaining weight.