The study was conducted to evaluate the effect of Twin-block appliance cephalometrically and to provide an insight into cephalometric variables that may contribute for final lip position outcome in treated growing Class II malocclusion patients with mandibular deficiency. Material and Methods: Fourteen patients (9 males, 5 females; mean age, 11.4 ± 2 years) with skeletal class II mandibular deficiency were treated using Twin-block appliance to enhance mandibular growth. The pretreatment photographs and cephalometric radiographs were obtained for every patient at the beginning of treatment (T0). All patients were treated and followed up for 9 months and then new records were obtained. lip position changes was evaluated from the lateral cephalogram. Results: Twin-block treated patients showed a significant effect on upper lip to E-line, U1\SN, SNA, ANB. The upper lip was retracted 1.8 mm to E-line while the lower lip retracted 0.78 mm to E-line. Upper lip position in relation to E-line was positively correlated with ANB angle and with Lower lip position in relation to Ricket’s E-line. Conclusion: Twin-block has a positive effect on the facial esthetic which could be explained by the significant retraction of upper lip in relation to E-line