Abstract: With the technological advancement, excessive screen times among young children are becoming a widespread issue and had an adverse effect on health. Therefore, it's crucial to raise mothers' knowledge regarding safe screen time. Aim: The study aimed to evaluate the effect of educational guidelines on mothers' knowledge and practice regarding using of screen viewing time on their children developmental health. Methods: A quasi-experimental research (one group pre/post and follow up) design was used to conduct the study. Setting: This study carried out in the nursery schools at Fayoum governorate. Subjects: A non-probability purposive sampling approach was employed, which specifically selected 120 mothers and their children under six years old. Tools: Tool (I) A structured interviewing questionnaire, included three parts: Part (1): Personal characteristics of mothers and their children. Part (2) assessment of the negative effect of screen viewing in children's health. Part (3) assessment of mothers' knowledge that related to screen viewing. Tool (II): Reported practices of mother's checklist, it consisted of two parts: Part (1): practice regarding screen viewing and part (2) practice regarding healthy developmental activities of the children. Tool (III): Developmental checklists, to assess children's developmental health milestones. Results: There was a highly significant improvement regarding mothers' knowledge, reported practice and children's developmental health milestones between pre assessment and post implementation of the Educational Guidelines as well as in follow-up assessment with P=0.000. Conclusion: Implementation of the Educational Guidelines were effective in enhancing mothers' knowledge and practices related to screen viewing time and in mitigating the negative effects of screen viewing on their children's developmental health. Recommendation: Continuous health education program for mothers about safety of screen viewing exposure is needed.