Background: Atelectasis is described as lung collapse that affects one or more regions of the lung. It is characterized by reducing lung volumes, altering the ventilation/perfusion ratio, and being one of the causes of delaying the weaning from mechanical ventilation.The aim of the present study was to investigate the effect of thoracic block technique on atelectasis in children on mechanical ventilation. Patients and methods: A total of 44 children with unilateral atelectasis on mechanical ventilation, from both gender and their ages ranged from 6 months to 4 years, were recruited from intensive care unit (ICU), Abo-Elreesh Hospital, Cairo University. Patients were randomly assigned into two groups; Group A included 22 children who received a designed chest physical therapy program,and Group B included 22 children who received a designed chest physical therapy program and thoracic block technique. Results: A significant increase in the percentage of totally improved lobes of lung in chest X-ray in both groups; the increase was significantly higher in patients who received a designed chest physical therapy program and thoracic block technique than patients who only received a designed chest physical therapy program (P=0.04). There was no significant difference in oxygen saturation index between groups post treatment (P=0.38). Conclusion: Applying thoracic block technique on mechanically ventilated children with atelectasis can produce a significant increase in the percentage of totally improved lobes of lung.