Background: Bone Marrow Aspiration (BMA) is an essential invasive technique for diagnosis, examinations or transplantation. BMA should be carried out by trained individuals who are aware of the indications, contraindications and complications of the procedure. Aim of the study: This study aimed to evaluate the effect of the designed nursing guidelines on nurses' performance regarding care of children undergoing bone marrow aspiration. Research design: Quasi experimental design was utilized. Sample: A convenient sample of all available nurses (30) working in Hemato-oncology department at Benha Specialized Pediatric Hospital and purposive sample of 60 child undergoing bone marrow aspiration. Tools: Two tools were used. Tool I: A structured interviewing questionnaire to assess personal characteristics of the studied nurses, characteristics of children, and nurses' knowledge regarding cancer and bone marrow aspiration. Tool II: Observational checklist to assess assess nurses' practice regarding care of children with cancer undergoing bone marrow aspiration. Results: There was a highly positive statistical correlation between total knowledge and total practices scores of the studied nurses at pre and post-implementation of designed nursing guidelines. Conclusion: The designed nursing guidelines were effective in improving nurses' knowledge and enhancing their practices regarding care of children undergoing bone marrow aspiration. Recommendations: Sustainable update of nurses' knowledge and practices regarding care of children undergoing bone marrow aspiration and continuing in- service educational programs should be designed and implemented to motivate nurses in achieving high competent care.