Oral appliances are increasingly advocated as a treatment option for obstructive sleep apnea (OSA). However, it is not clear how the different designs influence treatment efficacy in children. The aim of this study was to investigate the effects of twin block (TB) appliance on children with OSA and mandibular retrognathia. A total of 46 children (31 males, 15 females, aged 9.7 ± 1.5 years, BMI: 18.1 ± 1.04 kg/m2) diagnosed with mandibular retrognathia and OSA by polysomnography (PSG) and with no obesity or adenotonsillar hypertrophy were recruited for the study. Patients in the treatment group were instructed to wear the twin block oral appliance full time for an average of 10.8 months. The efficacy of treatment was determined by monitoring the PSG and cephalometric changes before and after appliance removal. Data were analyzed using paired t test. Results showed an improvement in patient's facial profile after treatment with the TB appliance. The average AHI index decreased from 14.08 ± 4.25 to 3.39 ± 1.86 (p < 0.01), and the lowest SaO2 increased from 77.78 ± 3.38 to 93.63 ± 2.66 (p < 0.01). Cephalometric measurements showed a significant increase in the superior posterior airway space, middle airway space, SNB angle and facial convexity which indicate an enhancement in mandibular growth, and reduction in the soft palate length. This preliminary study suggests that twin block appliance may improve the patient's facial profile and OSA symptoms in a group of carefully selected children presented with both OSA and mandibular retrognathia symptoms.