Asymmetric masticatory functions or occlusion are important in the development of facial asymmetry. In orthodontic treatment, severe skeletal asymmetries are preferably treated with a combination of orthodontics and orthognathic surgery, while mild mandibular and dental asymmetries with functional mandibular deviation are treated with nonsurgical orthodontic treatment combined with a splint or an implant anchorage, which result in alterations of the occlusal plane and condylar position. A few reports show that morphological changes of mandible in adults by surgical orthodontic treatment bring about alterations in muscle activities. However, it is poorly understood that the improvement of the mandibular position in adults by nonsurgical treatment induces the functional adaptation of masticatory muscles to the craniofacial complex. The purpose of this study is to evaluate the alteration of the masticatory muscle function after the nonsurgical orthodontic treatment for mandibular lateral deviation. This study was conducted with adult patients with mandibular lateral deviation that had been improved by orthodontic treatment. Skeletal changes were measured using lateral and frontal cephalograms and positions of the temporomandibular joints were evaluated with magnetic resonance image (MRI) or computed tomography (CT) images. Activities of masticatory muscles (the masseter, temporal and digastric muscles) were compared between pretreatment and posttreatment by electromyography (EMG). The right and left difference was expressed by an asymmetry index (AI) {(right-left)/left, %}. This study suggests that the bilateral balance of masticatory muscle activities changes when mandibular deviation and cant of the occlusal plane are improved by nonsurgical orthodontic treatment for mandibular asymmetry.