Introduction: Breast milk is the best life insurance for children under six months, but those fed exclusively with this milk do not reach 40%, a reason to promote breastfeeding. Objective: To evaluate the effect of nursing care based on Kristen Swanson's theory about the knowledge and acceptance of breastfeeding in first-time mothers. Methods: Quasiexperimental research, with pre-test and post-test, carried out in the maternity service and combined accommodation of Augusto Hernández Mendoza-ESSALUD-ICA Hospital, in Peru, during 2016. The universe consisted of 30 first-time mothers. The information was obtained from the variables nursing care, level of knowledge and acceptance of breastfeeding practice, using a survey and the checklist. The intervention consisted in applying nursing care based on the Kristen Swanson’s model to mothers. The information was processed with distributions of absolute frequencies, percentage, mean, standard deviation, variance, minimum and maximum value. The association between variables was obtained using the nonparametric tests of Wilcoxon and McNemar, with the following decision rule: If P≤0,05 Ho is rejected. Results: The mothers' knowledge about breastfeeding changed between measurements before and after the intervention (z=-5.203, P<0.05). The proportion of mothers who did not accept breastfeeding decreased from 30%, in the measurement before the intervention, to 6.66% after the intervention. Conclusion: The application of nursing care based on the Kristen Swanson’s model was effective in improving knowledge and acceptance of the practice of breastfeeding in the first-time mothers why participated in the study.