Background: Breastfeeding is one of the most effective ways to ensure child health and survival and is a central part of the 2030 Agenda for Sustainable Development. Knowledgeable health care professionals play a vital role in protecting, promoting and supporting breastfeeding. Unfortunately, many of nurses lack even basic breastfeeding knowledge and skills due to lack of training along with inadequate breastfeeding education in undergraduate nursing programs. Aim of the study: to evaluate the effect of breastfeeding training using educational toolkit on primary health care nurses' knowledge, attitudes, and problem-solving skills. Methodology: A randomized educational interventional trial (pre-post) study involved 364 nurses (182 intervention and 182 control) working in primary health care facilities in El-Behira governorate, Egypt. The intervention group received breastfeeding educational program using educational toolkit consists of a group of active teaching methods and materials. The nurses' (intervention and control groups) breastfeeding knowledge, attitudes, and problem-solving skills were evaluated before the training, immediate post training and after 3 months. Results: Before the training, 83.0 % of the intervention group and 88.5% of the control group were having neutral attitudes and the majority of both groups (95.1, 96.7 % respectively) were having fair knowledge about breastfeeding. Moreover, 90.1% of intervention group and 85.2% of the control group were incompetent in breastfeeding problem solving. Meanwhile, after training significant improvement occurred among nurses in the intervention group. Conclusion: The present study revealed that training using educational toolkit has positive effect on primary health care nurses' knowledge, attitudes and problem-solving skills related to breastfeeding. Recommendations: provide continuous in-service training programs for nurses regarding breastfeeding management and establish national standards and guidelines for BF support in all primary health care facilities based on the ten steps for successful breastfeeding.