T win-block is a functional appliance which is originally developed by Clark, and usually used for the correction of class TT division l malocclusion. Twin-block composed of upper and lower bite-blocks that engage on occlusal inclined plane and interlock at 70° angle. Occlusal inclined plane is used to stimulate the growth of mandible forward and downward position into a functional correction of occlusal relationships. Twin-block has got a simple, aesthetic and functional design for full-time wear and free the patient from the restriction of normal activity such as eating and speaking. This appliance is an effective appliance for the correction of class II malocclusion with mandibular skeletal retrusion by combined the dentoalveolar and skeletal change. The changes include greater increments in total mandibular length, reduction of overjet and overbite, correction of molar relationships, maxilla incisor retroclination and mandibular incisor proclination. The duration of twin-block therapy, on average, is 14 months. The optimal timing for twin-block therapy of Class II malocclusion is at the maturation of cervical vertebrae stage 2 (CVMS 2), which is concurrent with the onset of the peak in mandibular growth.