This study was conducted to determine the electromyographic features in the perioral muscles of class II division 1 malocclusion with incompetent lips, and to grope the correlation between its activities and craniofacial morphology. In this study, 14 subjects with class II division 1 malocclusion with incompetent lips(mean age of 20.5 years) and 20 subjects with normal occlusion(mean age of 23.9 years) were investigated. Electromyographic data were recorded from orbicularis oris, mentalis, buccinator and suprahyoid muscles durig rest lip posture, lip position at sealing, maximum sealing, maximal blowing, maximal biting, sipping milk, sipping and swallowing milk, chewing gum, masticating almond, swallowing almond and phonation utilizing the Medelec MS-25 electromyographic apparatus. Lateral cephalometric radiographs were taken with the mandible in intercuspal position on all subjects. All data were recorded statistically processed. The findings of this study can be summerized as follows: 1. In class II division 1 malocclusion with incompetent lips, the overall augmentations of perioral muscle activities during various functional movements set for lip sealing were manifested and particular swelling in mentalis activity at rest was detected. 2. On the other hand remarkable diminution of upper lip activities at lip sealing movements was drawn. 3. In Class II division 1 malocclusion with incompetent lips, negative correlations existed between the diversity of upper lip activities and upper incisor position and overjet as well in contrast to positive correla?tions in the lower lip. 4. It was suggested that the abnormal function of lower lip and mentalis muscle contributed somewhat the revelation of the characteristics of Class II division 1 malocclusion.