Background: Children routinely undergoing endoscopy experience severe procedural distress accompanying with negative feelings mainly anxiety. Purpose: to evaluate the effect of developmental psychological preparation on behavioral distress of children   undergoing endoscopy. Research design: A quasi experimental design. It was conducted at endoscopy unit of National Liver Institute Menoufia University. Sample: A convenient sample of 78 children who had liver diseases and undergoing endoscopy. Instruments: four data collection instruments were used; Social characteristics structured questionnaire, Physiological evaluation likert scale, Observational scale for behavioral distress (OSBD) and Pediatric End Stage Liver Disease (PELD) Score. Results:  there was highly statistical significant difference in behavioral distress between study and control group on posttests (p<.001). Conclusion:   children in the study group who are engaged in developmental psychological preparation exhibited lower behavioral distress level on posttests than patients in the control group. Recommendations:  The study recommended that developmental psychological preparation should be importantly integrated in endoscopic units as a routine care to lower negative consequences of this stressful experience.